First of all, when you want to find out what a command does, try typing in man command or info command (where you would replace command with the command you are trying to learn about).
Locate is a sort of find program that helps you to find various files on your computer. For example, if you are trying to find a file named mydoc.txt, but you can't remember where you put it, you could type in locate mydoc or locate .txt (yes, just part of the file name will work).
The problem it seems you are having is that your normal users aren't allowed to use the startx command. Chmod changes the 'permissions' on the file so that they are allowed to.
Another thing that might cause your problem could be that normal users don't have the command startx in their 'path' (where the computer goes looking for the program you told it to run). You can either type in the full path to it each time you want to run it (/usr/X11R6/bin/startx on my system), or add the directory containing startx (/usr/X11R6/bin) to each users path in the .bash_profile in their home directories.
Hope this helps.
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