A wood block is a small percussion instrument. It is made of wood and hit like a drum. The woodblock is hollow inside so that it makes a big, resonating sound. It can come in different forms, but many modern woodblocks are shaped like a brick.
It is sometimes called a Chinese block because it has Chinese origins.
The sticks used to hit the woodblock can be snare drum sticks or xylophone beaters. It makes a hard, high sound. It can often be heard in large orchestras, e.g. in Bernstein's West Side Story.