Permissions go in the order:
user group others
read (r) = 4 100
write (w) = 2 010
execute (x) = 1 001
To combine permissions add them up.
For directories:
read = View files and sub-directories.
write = Create files and sub-directories.
execute = Can enter the directory. (what does this mean? cd?)
chmod can be used to change the permissions of a file or directory.
Giving read and write to the owner (4+2=6), and read to the group and others (4).
chmod 644 foo.sh rw- r-- r--
Give read and execute to the owner (4+1=5), read to the group (4) and nothing to others (0).
chmod 540 foo.sh r-x r-- ---
Give read and write to the user (4+2=6), nothing to the group (0), and read to others (4).
chmod 604 foo.sh rw- --- r--