(from MikvahUSA.org)
A Mikvah is a natural body of water or a gathering of water that has a designated connection to a pool designed specifically for immersion, according to the rules and customs of Jewish law. The above body of water consists of naturally collected water that was never stagnant or gathered by human means. It contains about 200 gallons of water.
WHAT IS UNIQUE ABOUT MIKVAH WATER?
Water is the primary source of all living things. It has the power to restore and replenish life. A Mikvah must be filled with living waters from a flowing source, such as fresh spring water, rainwater, or even melted snow. The water is kept under strict hygienic control, and is cleaned regularly.
WHAT IS THE PRIMARY USE OF THE MIKVAH?
It is used by women as part of the Mitzvah best known as Taharas Hamishpachah or Family purity.
WHAT IS THE IMPORTANCE OF THE MITZVAH OF MIKVAH?
The Mitzvah of building a Mikvah takes precedence over building a synagogue. A synagogue or Torah Scroll may be sold to raise funds for the building of a Mikvah. While the synagogue represents the community, the Mikvah is a symbol of the family.
WHAT ARE SOME OTHER USES FOR A MIKVAH?
Use of the Mikvah covers many elements of Jewish life. The Mikvah is an integral part of conversion to Judaism; Mikvah is used for the immersion of certain new pots, dishes, and utensils. Mikvah is also used by men on various occasions: Erev Shabbos, Holidays, and a groom on his wedding day.
Mikvah is used by some to immerse the entire body of the deceased. This is the purification rite of a Jew before the person is laid to rest and the soul ascends to heaven.
But the most important use of the Mikvah is by women as part of the larger framework best known as Taharas Hamishpachah (Family Purity).
For a more detailed explanation of a Mikvah and its many uses, please click here.