Should be lots of good info on the net, but here are the very basics:
For any given file or folder, there are 3 'people-categories' for whom there may be permissions: USER, GROUP, and OTHER. USER means the owner of the file. Group means what 'group' of users who might access the machine, and OTHER means anyone on planet earth (Any user, any Group) who is using the machine.
For each of those 3 caterories there are 3 BASIC permissions:READ, WRITE, and EXECUTE.
READ means those people allowed, may READ the file. WRITE means those people allowed, may WRITE TO or CHANGE the file. And EXECUTE means those people allowed can EXECUTE or RUN the file (if it's a script or a binary, for example).
So, for any given file or folder, you must set WHO is allowed to do WHAT with a given file.
Search LQ here for things like PERMISSIONS, SUID, ACCESS, and stuff like that. There are other threads around with lots of info.
NOTE: These were the BASICS. There is also SUID, GUID, and STICKY which are three 'extra' special permissions, but that's a long story, which you will get the hang of if you read a bit.
Also, in a console, type 'man chmod' or 'man chgrp' for further info. For the most part, each permission is independant of each other. For example, a file can be USER(root)=RWX, GROUP(humans)=RW, and OTHER(anyone)=RX. (These would be rather silly settings mind you
) but they would mean that the ROOT user could read, write, or execute the file. Anybody in the group 'humans' could read or change the file, but not execute it. And anyone at all could read or execute the file. As I said, stupid permissions, but it's only an example.