A redstone repeater can be used to “repeat” redstone signals back to full strength, delay signals, prevent signals moving backwards, or to “lock” signals in one state
A repeater can only be placed on opaque blocks (dirt, stone, etc., but not glass, leaves, etc.), or on top of upside-down slabs, upside-down stairs, and hoppers. To place a repeater, use the Place Block control.
A redstone repeater has a front and back – the arrow on the top points to the repeater’s front. A repeater also has two small redstone torches on its top – the color of the torches indicates whether its output is on (dark red when off, bright red when on) and the distance between them indicates the delay the repeater adds to the signal transmission.
A repeater is 0.125 (1/8) blocks high.