A mikvah is a ritual pool of water made of naturally collected rain water. The main use of the mikvah today is by women after completion of their menstrual cycle.
The Torah teaches us that when a woman menstruates, a potential life has passed through her body. Life, in each of its forms, contains kedusha (sanctity or holiness.) When kedusha is removed from a place, what’s left in its place is Tumah, or impurity. The way we cleanse impurity is through immersion in a mikvah. The waters of the mikvah are derived from rainwater which is water from a natural source. These waters are like the waters of Creation and are akin to the amniotic fluid from which the world was created. When a woman has experienced the loss of potential life she needs to be reconnected to the source of all life which was the creation of the world. Immersion in the mikvah removes her impurity, thus allowing her, through purity to reconnect to life, and to live-giving activities.
In the time of the Temple, mikvah was used by both men and women for different occasions of achieving ritual purity. Nowadays the main use for women is for nidus, menstruation, and for men, general purification. Mikvaot are also used for conversions.