It is a mounting arrangement found on the top of the camera bodies for mounting external flashes, commander modules, GPS modules etc. The hot shoe is generally found in a standard form and the various compatibility issues arise due to the different firing voltages which trigger them.
One may not find daily use of the hot shoe but it sure is an indispensable bit of the camera.
Tell me in the comments what do you generally mount on the hotshoe!
Last time, it was Hyperfocal Distance.