זב מבשרו, our sages understand this as looking like a mixture of seminal fluid and a discharge which is the result of that organ being diseased.

רר בשרו את זובו, the word is similar to the expression ויורד רירו על זקנו “his spittle ran down his beard” in Samuel I 21,14 when David pretended to be mentally disturbed in order to save his life. The flesh of the person afflicted drips with this discharge.

או החתים, or that at the same time the discharge is sticky, congeals and adheres to part of the flesh of the afflicted person.

אשר ישכב עליו, even if the (healthy) person lies under a stone not touching the bedstead of the afflicted person at all. The zav’s ritual impurity is still conferred upon him. The common denominator of the wide-ranging ability to confer impurity of zav, zavah, and niddah, the menstruating woman, are that they confer their own severe degree of ritual impurity to anything which lies underneath them even though not in physical contact with them. The only exception to this rule is if said afflicted party sits or lies on something that does not normally serve as a place to sit on or to lie on.

. וידיו לא שטף במים, the zav did not immerse himself in a ritual bath.. This is the way the sages interpret the words above (compare Rashi) Personally, I believe that the plain meaning of these words above is that the person experiencing these drippings and sticky semen adhering to his flesh mentioned in verse 3 has not yet removed this from himself. The Torah uses an elegant way saying “he has not washed it off with water,” basically, it refers to a removal by wiping or scraping his skin including his glans. The lesson here is that a simple immersion in a ritual bath, if not after the removal of the offending material, does not accomplish its purpose of ritually cleansing the afflicted party. This has been explained in detail in Niddah 15 where the words in our verse are understood as “although he had previously immersed himself he still remains ritually impure, seeing he had not first cleansed himself physically. Spiritual cleanliness presupposes physical cleanliness.” Anything or anyone touching the offending fluid (until it has thoroughly dried, congealed) becomes contaminated ritually. The expression שטיפת ידיו במים, loosely translated as “immersing one’s hands in water,” is used here in a sense similar to Proverbs 30,20 where the author describes the unfaithful wife as “wiping her mouth” after indulging in illicit sex, proclaiming that now that she has “wiped the offending residue of her indulgence,” she has not committed any sin. Solomon describes the scenario sarcastically, of course. A similar scenario occurs in Judges 3,24, where Eglon’s servants assumed that their king was still busy using his private toilet and this was why he did not open the door for them. The expression used there was מסיך הוא את רגליו, an elegant way of referring to someone urinating or excreting. Here the Torah uses the elegant way of referring to the sticky mess still adhering to the afflicted party’s body as ידיו לא שטף במים, “he did not rinse his hands in water.” In other words, he had not yet gotten rid of the offensive material causing the ritual impurity in the first place. The expression is used without recourse to elegance in Leviticus 6,21 where the words ומורק ושוטף במים mean: ”it has to be purged and thoroughly rinsed out in water.” The fatty substances of the sin offering in that vessel have to be cleaned out. In Isaiah 28,2 we also read about מים כבירים שוטפים, “the destructive force of mighty waters.” It sweeps away anything before its force, “cleans” it out. In Job 14,19 we also encounter the expression שטף in that sense, i.e. “torrents wash away earth.”

דם יהיה זובה, the reddish colour causes the ritual impurity here, not the whitish colour, suggesting seminal fluid.

'ואם על המשכב הוא וגו, if some sits or lies on something even indirectly in contact, מדרס, with the afflicted person chair or bed, etc., he will still become ritually contaminated by such indirect contact בנגעו בו יטמא עד הערב, even if the “contact” was so indirect as instead of sitting on the contaminated item one sat on a saddle supported by the contaminated item, the ritual impurity lasts until evening when immersion in a ritual bath removes it. He is then, however, not of a sufficiently severe degree of ritual impurity to contaminate clothing (even his own) over and beyond his body. In other words, in this kind of scenario the Torah did not add that he has to wash (immerse) his garments in order to become ritually pure again. (as it had done in verse 21 and 22, for instance)

ואשר היא יושבת עליו, our sages (in Torat Kohanim zavim 4,18 understand this as referring to the relatively low level of ritual contamination called טומאת מרכב, see beginning of this verse).

אשר ישכב עליו יטמא. This degree of ritual contamination is sufficient to confer further contamination on food and drink coming into contact with it. The severity of contamination of someone lying on top of a menstruating woman is equivalent to that of someone below a male afflicted with the disease of being zav. This is spelled out in Niddah 32.

בלא עת נדתה, after the days of her menstruation have passed, as explained in Niddah 73 A woman is not described as being a zavah so that she would have to bring an offering after being healed unless the seven days during which she was a menstruant or expected to be a menstruant have passed. If after that period during the next 11 days she sees the symptoms of a zavah for three consecutive days she will require to count seven consecutive days during which she is free of these symptoms in order to be able to become ritually pure again. On the eighth day she is ready to offer her two bird offerings, one as a sin offering the other as a burnt offering.