Mikveh immersion is the final step in the process of becoming tehorah. This is a Torah law based on the authoritative interpretations of several Biblical verses. These include Leviticus 15:18, "and they shall bathe in water," and Numbers 31:23, in which a mikveh is referred to as "the waters of niddah." A woman in niddah who immerses following her seven blood-free days becomes tehorah and is permitted to resume physical intimacy with her husband.
Every woman and every immersion is unique, and there is an infinite variety of valid mikveh experiences, ranging from the prosaic to the transcendent.
Mikveh night is an opportunity for the couple to focus on each other and to rekindle excitement within marriage. Sometimes, mikveh night naturally becomes a second, or a fiftieth, honeymoon. At other times, some advance discussion of needs and expectations can help in planning a positive evening for both husband and wife.
Much has been written about the inner meaning of mikveh. Two excellent books on the topic are Total Immersion: A Mikvah Anthology (by Rivkah Slonim, Urim Publications, 2005) and Waters of Eden: The Mystery of the Mikvah (by Rabbi Aryeh Kaplan, NCSY/Union of Orthodox Jewish Congregations of America, 1976).