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.