Chmod is a UNIX and Linux command for setting file or directory permissions. It is a confusing topic until you learn it, but it is needed if you plan to work with UNIX or Linux web servers.
There are three different possible user levels, each with three different possible settings.
The three user levels are Owner, Group, and Other.
The three possible settings are Read, Write, Execute.
The three settings are given numeric values. Read has a value of 4. Write has a value of 2. Execute has a value of 1.
You add together the numbers for the permissions you want. For example, for Read and Write permissions, you Chmod 6, since Read (4) + Write (2) = 6.
You would need to do that for each group. For example, Read + Write + Execute permission for Owner, and Read permission for Group and Other, would be Chmod 744
If you want everyone to have full access to the file or directory, you would Chmod 777. Which is Read/Write/Execute for all Owner/Group/Other users.
It may come in handy.