A woman becomes a "niddah" through the discharge of uterine blood. There are two separate areas where this is important.
In the time of the Temple and a while later, when people were careful with the laws of ritual purity, she would have to guard against touching ritually clean foods, so as not to make them ritually impure. Incidentally, a man can also have a discharge which will give him a similar legal status, and this discharge is called "zivah," while he himself in this state is called a "zav."
Secondly, a niddah is prohibited to her husband with a strong prohibition of prohibited relations, and if they cohabit, they both become liable to being cut off from the spiritual source.