As it says, it _is_ really simple: * "chmod octal cheat sheet":http://kernelhack.com/?p=22

	user	group	all
7	rwx	@rwx@	rwx
6	@rw-@	@rw-@	@rw-@
5	@r-x@	@r-x@	@r-x@
4	@r--@	@r--@	@r--@
3	@-wx@	@-wx@	@-wx@
2	@-w-@	@-w-@	@-w-@
1	@--x@	@--x@	@--x@
0	@---@	@---@	@---@

One example is the permission @755@ that translates to @rwxr-xr-x@ giving the owner @rwx@, and anyone else can only read and execute it. This is useful for scripts, that you want outsiders to be able to read and execute, but not modify.