MC4 connectors are single-contact electrical connectors commonly used for connecting solar panels. The MC4 and compatible products are universal in the solar market today, equipping almost all solar panels produced since about 2011.
The MC4 system consists of a plug and socket design. Oddly, the plugs and sockets are placed inside plastic shells that appear to be the opposite gender - the plug is inside a cylindrical shell that looks like a female connector but is referred to as male, and the socket is inside a square probe that looks male but is electrically female. The female connector has two plastic fingers that have to be pressed toward the central probe slightly to insert into holes in the front of the male connector. When the two are pushed together, the fingers slide down the holes until they reach a notch cut into the side of the male connector, where they pop outward to lock the two together.