It's mostly a Master/Mistress reference. The feminine word for Master is sometimes "Mistress", which signifies a woman in control of you.
It's mostly a BDSM term, but whether it's light or heavy depends on the cam girl and the viewer. There are some mistresses into gentle femdom, and others into some very extreme control play, like findom, consensual blackmail, and dominatrix/humiliation stuff.
To each their own.