It is composed out of a "plank plate" on the top and a dark grey base with a small black dot on the base that connects to Wire.
It is activated when a player steps on it, but also can be triggered by blueprints, wood (milled or not), wires, placeable items, loose items and dropped axes. it can be useful for opening doors when a player enters their house. It can also be used to switch a Conveyor or a Chop Saw when a plank or log activates it.
It also makes great timers for blinking lights, a constantly active Chop Saw, or maybe a storage when you place an axe or an object on the Pressure Plate and something lights up so you know where to go.