The term Mistress has two meanings.
In BDSM, Mistress is the female equivalent of Master. See also dominatrix. A person's Mistress is typically the one in charge or the owner. A bottom shows respect to the Mistress by addressing her formally, e.g. "Yes, Mistress". "Ma'am" or madam are similar terms that show respect, e.g. "Yes, ma'am". A bottom learns which term is required by the Mistress.
In a household, a male submissive may address his top in a formal manner using Mistress and the shorter Ma'am. Such spoken words reinforce submission to her authority. In domestic discipline, the Mistress may spank her bottom at will. A casual form of address and swear words in a spanking scene are inappropriate. Bottoms continue to honor their Mistress during a session to help them accept their discipline, e.g. "one more, Mistress" after each stroke.
The title Mrs. is an abbreviation of Mistress, and the titles Miss and Ms. are also derived from it.
A female dominant on a throne, with slaves. Illustration by Jim Black (Luc Lafnet), 1929.
Outside a BDSM context, a mistress is a female lover, i.e. a woman who has a long-term sexual and possibly romantic relationship with a man, in return for support and, generally, payment from the man to the woman.