In Judaism, to be tehorah is to be in a state of cleanliness or purity.
There is a particular focus on the period when a woman is niddah, or unpure, due to her monthly cycle. There are prohibitions against sexual activity during this time. It is not permitted for a man and his wife to share the same bed while she is niddah. There are a multitude of other restrictions on permissible behavior which can best be explained by a Rabbi especially trained in these matters.
At the onset of menses there are a minimum of five days which are counted as days of bleeding, whether there is actual flow or not. Following the woman's cycle there must be seven days free from menses. Then it is necessary that a woman immerse herself in a kosher mikveh, finalizing the cleansing process. Having completed these 3 phases, the woman again becomes tehorah.
Sources:
http://www.yoatzot.org/topic.php?id=20
http://www.foreveryjew.com/women/nidah/1.html