This mitzvah prohibits having sexual relations with a mother and her daughter. (It does NOT mean “at the same time,” though that is certainly likewise prohibited.) This mitzvah applies specifically where the man had married either the mother or daughter and then subsequently has relations with the other one. (We see that this mitzvah only applies in a case of marriage from the words “lo sikach” that appear later in this verse. These words literally mean “do not take,” but it’s the Biblical phrase meaning to take in marriage. We’ll actually reach these words in the verse when we get to Mitzvah #205.)
This scenario is actually a prohibition against having sex with one’s stepdaughter or mother-in-law. If the man marries the mother, the girl becomes his stepdaughter; if he marries the daughter, the mother is his mother-in-law. We’ve discussed enough incestuous relationships by now to be able to see why such sexual encounters should be prohibited as inappropriate. As with other arayos, this prohibition lasts in perpetuity, even after the marriage is dissolved by death or divorce.
Rabbinically, it was enacted that if a man had relations out of wedlock with a woman, he would not be permitted to marry any of her immediate relatives (mother, daughter, sister) but only so long as the woman with whom he was intimate was still alive. The reason for this is that it’s inevitable that he’s going to be seeing her family and hanging around his former lover; no good can come of that! (See Talmud Yevamos 97a.)
In most of the cases of incestuous relationships, all willing participants may be subject to the death penalty. Here, only the man and the second woman can be liable. The first woman–that is, the one he married–did nothing impermissible. The death penalty could only be imposed if both women were still alive. If a man slept with the second after the first had died, it was still prohibited but not subject to capital punishment. (This is derived from Leviticus 20:14.)
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Yevamos on page 97a and in Sanhedrin on 76a-b. It is codified in the Shulchan Aruch in Even Ha’Ezer 15. This prohibition is #337 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #121 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.