It is used to terminate one or more processes by sending signals to them. The default signal is the terminate signal(9). With the option you can send a different signal. The option is a number (0-15) that is preceded by a hyphen.
kill [option] PID-list
If you type in the command kill 1527, the process with the PID of 1527 will be terminated. To terminate all of your jobs and log yourself out you can type kill -9 0. If you are a superuser this would bring down the system, so be careful.