Whenever a command is invoked it is passed as its first argument what is known as the prefix argument. The prefix argument is always either an integer or nil. When a command uses this value it is usually as a repeat count, or some conceptually similar function.
This function returns the current value of the prefix argument. When
set with setf, the new value becomes the prefix argument for the
next command.
If the prefix argument is not set by the previous command then the prefix argument for a command is nil. The prefix argument is not cleared after a command is invoked due to a transparent key binding.