If you want to save code on the disk, it is useful to learn about changesets. See http://minnow.cc.gatech.edu/squeak/uploads/changesets.pdf
Changesets are pretty easy to use once you get used to them, but they are easy to misuse when you haven't used them much. So, practice! Play around with them until you are comfortable with them.
You can use changesets to manage code in a multiperson project, but if you are on a real multiperson project then you should use Monticello, instead. In fact, you might want to use Monticello even for a single person project, but if you are getting started with Squeak it is probably easier to learn about changesets..
-Ralph Johnson