A counting semaphore is a synchronization tool used in computer science to manage access to a shared resource by multiple processes. It allows a specified number of processes to access the resource simultaneously. The semaphore maintains a count that represents the number of available resources, and processes can increment or decrement this count as they acquire or release the resource. When a process wants to access the resource, it checks the semaphore's count. If the count is greater than zero, the process decrements the count and proceeds. If the count is zero, the process must wait until another process releases the resource, thereby incrementing the count again. This mechanism helps prevent race conditions and ensures orderly access to shared resources.