A Weaver rail mount is a system to connect telescopic sights and other accessories to rifles, shotguns, pistols, and crossbows. It uses a pair of parallel rails and several slots perpendicular to these rails.
The later Picatinny rail, developed by the US military, is a development of the key concepts of the Weaver system, and they are partly compatible.
The slots on a Weaver system are primarily used as a clearance of the locking screw that tightens the clamp to the rail. Some Weaver-type accessories have a bar that fits inside the machined slots on the rails while many others do not.