This is the point in a pregnancy when the mother can feel the fetus move. It’s used to denote the first time this happens, so it’s the quickening, meaning it’s a singular event that either has or hasn’t occured.
Wikitionary states that an archaic definition of “quick” is “alive.” Before the advent of modern medicine, the first feeling of fetal movement would be the first proof for a woman that she was pregnant.
The quickening has been used in many legal contexts over the years to determine if a woman is pregnant. It’s mentioned in many laws about abortion, and was sometimes used to determine if a woman could be punished for a crime. (Though, it’s hard not to notice that it’s a very personal thing. Only the pregnant woman knows if quickening has occurred, so she could say it had or hadn’t, depending on what would give her the most benefit.)