This command is part of the posix extensions and needs to be imported to be available to the shell.
See Posix Commands
|-f,-force||Force move even if target exists or is readonly.|
If there are 2 or more source files or the target is a directory then files are moved to the target directory.
If there are exactly 2 arguments and the target does not exist or is not a directory then the source ("file") is moved (renamed) to the target file.