Editing covers a lot of different topics. There is reshaping, resizing, coloring design differently, combining designs, etc. These are all "editing". So editing is realy just a "catch all" for making a change to the design.
In Getting Started > lesson GS14-01 and GS14-02 there are some basics to getting started. But in almost every video we do, it has some type of editing.
I think it would be best to determine what you want to do, and then to find the video of that topic. But have a look at the GS14 section first.