The process id (PID) is like a serial number that uniquely and succinctly identifies an instance of a program that is running. (More than one instance of a program might be running at the same time.)

kill is an application that can be run by simply typing kill in a terminal. kill PID is like asking a troublesome guest with ID # PID to leave.

-9 is an option that modifies the behavior of the application kill. kill -9 PID is like cutting the head off a troublesome guest, possibly making a mess, and increasing risk of harm to yourself.