tick.ece.utexas.edu% ls -l -rwxr--r-- 1 dna students 12 Sep 6 13:35 ExampleFile tick.ece.utexas.edu%The ls command displays a listing of the files in the current working directory. The -l flag tells ls to provide a "long listing," which includes the permissions associated with the file. The permissions show up on the left and in this case they are: rwxr--r--.
An r indicates read permissions, a w write permissions, an x execute permissions and a - indicates a permission isn't granted. The permissions are broken into three sets of three: moving from right to left, the first set: rwx are the read, write and execute for the user; the second set: r--, are the read write and execute permissions for the group; and finally, the third set: r--, are the read, write and execute permissions for the public. For this set of permissions rwxr--r--, the user has read, write and execute permissions, but the group and the public have only read permission.
tick.ece.utexas.edu% ls -l -rw-r--r-- 2 dna students 12 Sep 6 13:35 assemble.c tick.ece.utexas.edu% chmod 600 assemble.c tick.ece.utexas.edu% ls -l -rw------- 2 dna students 12 Sep 6 13:35 assemble.c tick.ece.utexas.edu%The 600 is an octal representation for these permissions: rw-------. It's left as an exercise for the reader to explain this octal encoding.
For further explanation see the man pages for ls and chmod or contact a TA.