The © character is used for adding comments in a program: everything after © is ignored by the calculator for the purposes of actually running the program, so it's a good way to make a note to yourself about what a part of your code does. This is especially helpful if you're going to be reading the program later when you don't quite remember what you were doing.
:If ok=0 © If the user pressed ESC : Stop
There are other situations you might use comments in. For instance, you might make a rough sketch of your program and add comments about the code that has yet to be filled in:
:If key=264 Then : © add a confirmation dialog here later : Exit :EndIf
Yet another use of © is to "comment out" lines of code that you might need later, but want to ignore for now — this is better than deleting the code, since you don't have to rewrite it later.