What are signals that inform the Operating System that something has happened?

Messages or values in registers. Messages are passed between programs and the operating system to tell something else what to do. For example, when you click on a program, the computer does not directly run the program's code. The operating system receives a message telling it to run the specified program and does so. It makes note of this in internal registers, which keep track of what program is executing and what line of code is being executed.