Chmod is a command used to change file permissions. Given the right permissions, you can restrict or grant access to a file for specific groups or users. Here's the basic format: "chmod 755 myfile.txt" A file can have the following permissions corresponding to these numbers: Read = 4 Write = 2 Execute = 1 In order to combine permissions, you just have to add the numbers together (ie, READ & WRITE permission is a 6). A file has to give these permissions to three groups of people. For example, each digit in the "755" of the chmod command above represents one of these groups: Left-most Digit    = Owner Middle Digit         = The file's group Right-most Digit = All other users so in the command above, we gave the owner read, write, and execute permissions; the group of the file got read and execute permissions; and all other users got execute permissions. Hope that helps.