A splint is something that's used to keep a person's arm or leg straight and immobile while a broken bone is healing. If you break your wrist, you may need to wear a splint for a while.
Some splints are temporary, meant to keep an injured person's bones from moving until they can be examined by a doctor. EMTs and other emergency workers often use splints while transporting someone to a hospital. You can also use this word as a verb: "Let's splint your dog's leg and get her to the vet." A strip of wood can also be called a splint.