(NSFW) To what, exactly, does the term "Vanilla" refer?

I have heard the term “vanilla” used only to describe someone who is not at all into BDSM. Is this the only criterion? Are people who are sexually adventurous or wild in other ways considered vanilla? Does “vanilla” merely refer to someone who likes no-frills missionary or some other “basic position” sex?

Please help me to understand this term. Thanks collective!