Febfem (short for "female-exclusive bisexual female") is a bisexual or otherwise m-spec woman who chooses to only date women. It is different from a lesbian because one still experiences attraction to men, but does not pursue it. This includes women who have been traumatized by men and don't want to date them anymore, women who are politically against closely associating with men, and women who are significantly more attracted to women and therefore wouldn't be happy with a man. This label also can be seen as an alternative to bi-lesbians and other m-spec lesbian labels.
While it technically means a bisexual and m-spec woman who only dates other women, in practice it's almost exclusively used by transphobes (specifically TERFs) to mean a bisexual woman who will only date cis women and trans men.