A lens mount is basically the mechanical or electrical interface between the lens and the camera. The mount enables you to attach the lens to the camera. Mounts are used for cameras that let you interchange lenses. There are two types of common lens mounts, the screw-in types and bayonet versions. Most modern mounts are the bayonet style as the mechanical and electrical contacts need to be in the right position to line up with those on the camera's body.
Bayonet mounts usually have about three tabs around the base of the lens which attach into the mounting plate on the camera. Usually the tabs are made to make sure the lens can only be installed in one orientation. When you have inserted the lens you turn it slightly to lock it in place. A spring loaded gun will keep it secure and you can push this button to remove the lens.
Beware that most brands of lenses are only compatible for that brand of camera. Lens mounts are built into the lens.