That is that He commanded us to redeem the Hebrew maidservant. And that is His, may He be blessed, saying, "he must let her be redeemed" (Exodus 21:8). This redemption has many details and conditions and there are many regulations for it - and they have been explained in Tractate Kiddushin. And the law of the Hebrew maidservant is completely explained there. And in the Mekhilta (Mekhilta d'Rabbi Yishmael 21:11:1) they said in explanation of, "And if these three he does not do to her" (Exodus 21:11), "Designate (marry) [her] yourself, to your son, or redeem her." (See Parashat Mishpatim; Mishneh Torah, Slaves 4.)