Female domination, or Femdom, is when a woman is dominant during BDSM play. This isn't necessarily related to sexual orientation, as both straight and same-sex relationships can have Tops and Bottoms. Sometimes people implement this power dynamic in their lives 24/7, but much more often, the power play is limited to the bedroom.
Often the woman in charge has a strong and confident personality, but a man can also enjoy being submissive in bed precisely because he is excessively in control in daily life.
Like any part of BDSM, this needs to happen on a truly consensual basis, which means that boundaries are negotiated beforehand, a safe word is chosen. Both parties obviously must have a lot of trust in each other. FemDom might not play a part in every sexual encounter between people, rather the Mistress might emerge only in some sessions.
If Femdom sounds interesting to you, read more:
Bondage,
Submission,
Domination,
Sweet Pain