A binary semaphore is a synchronization tool used in computer science to manage access to shared resources. It can have only two states: locked (1) or unlocked (0). This simplicity makes it effective for controlling access in concurrent programming, ensuring that only one process can access a resource at a time. When a process wants to use a resource, it checks the state of the binary semaphore. If it is unlocked, the process changes the state to locked and proceeds. If it is already locked, the process must wait until it becomes unlocked. This mechanism helps prevent race conditions and ensures data integrity in multi-threaded environments.