The AND Gate is a Logic gate that will output power if it receives power from all of its inputs.
It has three inputs and one output. The three Redstone Torches on the back will go out when a input is applied to its 'channel'. When all are 'out' the output will light up and give out a redstone current.
The AND Gate by default has three inputs. Holding shift and right-clicking with a Screwdriver will cycle through disabling some of the inputs, allowing it to be used with just two. An inactive input is missing the red color from its side.
AND Gates can be used in circuits where something should only activate if it is powered by multiple sources. For example, a bunker may have a self-destruct mechanism, that will activate if 2 people both press a button simultaneously.
It can also be used to create a "safety lock". An example would be a room that can be remotely flooded with lava when a piston is activated by pressing a button. To prevent accidental activation, a lever and an AND gate could be implemented, requiring the lever to be pulled in addition to the button being pressed.It can also make a password lock were a few levers have to be pulled.