A cloak is anything that conceals or hides something, like an over-sized, dark raincoat you wear when you don't want your friends to see you're going to the movies without them.
As a noun, a cloak is usually a loose piece of clothing that you wear over your other clothes, like a cape or a gown. It especially refers to an outer garment that you might wear while traveling in order to protect your outfit or to conceal your identity. As a verb, to cloak is to conceal or hide something. If you were a famous person who wanted to go out alone, you might cloak your identity with a cloak.