Celestials are, as far as we've been shown so far, the universe's equivalent of gods. You know how Earth has its pantheons of gods--the Asgardians, the Olympians, etc.? Well, the Celestials are basically like them to a far, far, far greater level. They just move around the universe, messing with different races' development in an apparently random fashion (they're responsible for creating the Eternals, the Deviants, the Skrulls (as we know them), and I think they've played with the Kree's genes in the past). They're basically on the level of Galactus--some comics have portrayed them as easily more powerful than he is, some have portrayed them a bit less powerful, but they're always pretty high up there on the power scale. No one knows why they wear armor or what they really look like, and they never speak unless they take human form. They're a big cosmic question mark and they bring a lot of mystique to the Marvel universe. They're cool.