A mikvah is a ritual bath used by both men and women (separately, of course). For the man, it is an act of purification. For example, before the Sabbath and Yom Kippur, many men go to the mikvah. It is a way of preparing not just his soul, but his body, for the holiday that is about to come.
For the woman, visiting the mikvah creates a separation between the days that she kept herself from her husband, and the weeks she can be with him fully. In many ways, for men and women, the mikvah represents a separation between death and life; the waters of the mikvah restore each to a higher spiritual level.
The mikvah is such an important elements in Jewish life, Jews are commanded to build a mikvah even before building a synagogue. When away from home for any extended period of time, looking for the nearest mikvah can become major endeavor. This site – the Worldwide Mikvah Directory – can be a fantastic resource: http://www.mikvah.org/directory