A "Runnable" is an interface in the Java programming language that represents a task that can be executed by a thread. It contains a single method, run(), which defines the code that will be executed when the thread is started. By implementing the Runnable interface, developers can create tasks that can run concurrently, improving the efficiency of applications. Using Runnable is beneficial for separating the task's logic from the thread management. This allows for better organization of code and makes it easier to manage multiple threads. Runnable can be used with the Thread class or with ExecutorService for more advanced thread management.