The Buffer Gate is a Redstone Logic device. When placed, the input is nearest to the player. The other three sides are output only, making this gate a diode much like the Redstone Repeater. The three output squares will only emit a signal if it receives a Redstone signal from its input side, allowing for the creation of one way circuits.
UsageIt's better to use this device instead of the normal vanilla repeater, if you want to place it in a corner.
The Buffer Gate also can be used as a repeater with three outputs, as there is a slight delay between the time when a Redstone current reaches the buffer and when the buffer outputs a redstone current. This delay can be used for a "runway effect" with lights in a pathway.
It could also be used to split a Redstone current into three, great for compact redstone circuits.