permissions are represented by 3 digits

ugo => u=owner, g=group, o=otherw

1=execute, 2=write, 4=read

chmod 664 file => read and write access for the owner and group, read only for all others. 

 chmod 0 => removes all priveledges for all

chmod -R 755 => recursive, read+write+execute for owner, read+execute for group and others