The following quote from [Wikipedia](https://en.wikipedia.org/wiki/Signal_(IPC))
explains what signals are:

> Signals are standardized messages sent to a running program to trigger
> specific behavior, such as quitting or error handling. They are a limited
> form of inter-process communication (IPC), typically used in Unix, Unix-like,
> and other POSIX-compliant operating systems.
>
> A signal is an asynchronous notification sent to a process or to a specific
> thread within the same process to notify it of an event. Common uses of
> signals are to interrupt, suspend, terminate or kill a process.