A moonlodge is a place where a woman goes when she is menstruating. Several Native American tribes believe that a woman on her moontime or moon as menstruation is referred to, is in a powerful state of being and should be isolated and not allowed to cook food and do other tasks. The moonlodge is a distance from the regular home, and is used only for the purpose of this isolation.
Menstruating women are not allowed to participate in most Native American ceremonies, such as the sweat lodge and the sundance. This is not, as is believed by some, due to her "unclean" state, but is because a woman who's womb is open to the energy of the earth is very powerful and this power needs to be focused and managed.
Indians today that I know, keep the moonlodge beliefs alive to a certain extent. No longer do many women live apart from their families during this time, but some modern Indian women refrain from cooking food and participating in ceremony during menstruation.
The concept of a moonlodge has been used freely and loosely by the new age community, and many womens' spiritual gatherings feature a 'moonlodge' for women. These retreats usually have little to do with the original moonlodge tradition, but are used as a place where women can connect with each other spiritually.