זאת חקת התורה, in verse 14 the Torah already explains the purpose of the red heifer, i.e. which statute of the Torah is being introduced here.

פרה אדומה; this animal is required so that its ashes can be used to purify people who had become ritually defiled through direct or indirect contact with the body of a deceased human being.

לעיניו, in the presence of Eleazar.

עוד טומאתו בו, even though he had immersed himself in a ritual bath.

וכל אשר יגע על פני השדה, if one enters a closed area, roofed area in which the remains of the deceased are present one becomes ritually unclean, whereas if the remains of the deceased are in an open area such as a field, only direct contact with the deceased confers such ritual impurity.

והנפש הנוגעת, a third party who had had direct contact with someone had had direct contact with the deceased, תטמא עד הערב, will become ritually defiled only until the evening of that day (and can immerse himself in a ritual bath to regain ritual purity).