A clamp is a tool that's used to secure two things tightly together. When you repair a chair, it's best to use a clamp to hold the glued pieces together firmly.
Carpenters, woodworkers, and plumbers all use clamps when they want to hold things together while they wait for glue to dry or need their hands free. When you use this kind of device, you can also say you clamp something: "Clamp that board down for me before I turn the saw on." Clamp most likely shares a Germanic root with the word clam — *klam-, "to press together."