These small, two-pin, SPST momentary push buttons are meant for mounting to PCBs, but they can also be plugged into regular 0.1" breadboards. It is mainly used to close or open a circuit. It is 2 legs and one small button. After the button isn't pressed, the circuit is open (not connected) and if its pressed, the circuit is closed (attached). Notice that this button should not be used with voltages above 12 V, and it should not be used to change currents greater than 50 mA.