MOLLE is an acronym for Modular Lightweight Load-Carrying Equipment. The standard has military origins and allows customization of carrier systems. Molle fasteners are horizontal straps sewn into the equipment. Additional equipment is attached via loose straps threaded under/over the sewn-in straps.
MOLLE allows you to customize the carrying system with extra pockets and optimal weight distribution. You typically find molle mounts on belts, plate carriers and tactical vests. Common equipment includes holsters, magazine pouches and bags of various kinds.