What do "R" "W" and "X" mean?

I'm sorry for the really basic question here.

While editing the preferences of a file what do "R" "W" and "X" mean?

I am guessing Read Write and Delete... but I am pretty sure I am wrong.

Read, Write, eXecute.
For files eXecute means that you can run the file. For directories it means that you can change to the directory (make the directory current).