in the context of photography: that interesting metal slide-like thing you can find on the top of nearly all SLR. It works as a hardpoint for mounting a flash, and at the same time provides the flash with the synch signal that it needs to fire when the shutter opens.
A basic hotshoe simply short-circuits the outer metal brackets with the central post. Some hotshoes have additional contacts, meant for interfacing with dedicated flashes.

All hotshoes have the same measures, which means that (at least for basic functionality) you can mount almost any flash on any camera - with the exception (thank SharQ) of the evil Minolta brand, that uses a nonstandard hotshoe.