(u=user_owner, g=group and o=other or world)

The digits are the sum of the permission numbers for each category of individuals:

    4 = r, (read permission)
    2 = w, (write permission)
    1 = x. (execute permission)
    0 = - (no permission for this user)

The permissions granted the individuals in each category is the sum of the permission numbers.

Example:
To set read write execute for individuals defined as owner_user,
To set read no-write execute for individuals in the group
To set read no-write execute for all the individuals in the world
The calculation is as follows:

    Sum of permissions for u: rwx => 4 + 2 + 1 = 7
    Sum of permissions for g: r-x => 4 + 0 + 1 = 5
    Sum of permissions for o: r-x => 4 + 0 + 1 = 5

Which makes : 755

The command to accomplish this set of file permissions:

chmod 755 filename

Directories:

Permissions for directories.

read
    individuals can view the directory's contents
write
    individuals can create new files or delete files in the directory.
       (Note: an individual  with write access to a directory can
        delete files in the directory even without write permissions
        for the file.)
execute
    individual can cd into the directory. =================================================================