Could you explain what the use of -9 in a Linux command means?

In Linux, using -9 is usually associated with the kill command and means sending the SIGKILL signal to a process. This signal immediately terminates the process without the possibility of interception or graceful shutdown.

Example:

kill -9 1234

where 1234 is the process PID. It is a forceful way to stop a process when the normal kill (with SIGTERM) does not work.