Pressure Plates were introduced in the third Seecret Friday update, if connected to a wire (or placed adjacent to a block that requires electricity), they will power an object when stepped on, and will stop when stepped off.
There are currently only two types of pressure pads, wood and stone. Wooden pressure plates ones are made by lining up 3 wooden blocks in a horizontal matter, and stone pressure plates are made with smelted stone in the same fashion.
You cannot drop objects such as sand onto the pressure pads. However, throwing an item onto a wooden plate does trigger the plate. Stone plates however, are not affected by this action.
Stone and wooden plates can be recovered with a pickaxe. Both can also be received by destroying the block they are on.