Vanilla is the flavor for food — such as ice cream — that contains extract from the vanilla plant. This word also describes things that are boring or plain.
If something is called vanilla — such as coffee or a milkshake — it's usually flavored with vanilla extract or the seeds from a vanilla pod. Vanilla is the most common flavor of ice cream. This word has another, slightly insulting, meaning: a vanilla movie is plain and kind of boring. A vanilla song is on the blah side. Maybe it's because vanilla food tends to be white and plain that we use this word for other things that are bland.