Command: chmod
Usage: The chmod command sets the permissions of a file or directory. In Unix files and directories have three sets of permissions: owner, group, and other (everybody else). You are the owner. Of course you want to be able to read your own files, make changes, and run scripts. The read, write, and execute permissions control this. The group and other permissions are next. Normally, you don't want anyone else to make changes to your website! By default, this is set to read-only. People can go into your directories, but cannot change anything, or upload files there.
The most frequent reason to use this command is to make cgi scripts executable. You do this by typing chmod 755 filename at the command line. The 7 means read, write, and execute for the owner (you), while the 5 means read and execute, but not write, for the group and other.