Incremental saves are when a new save file is created; leaving the old one in the state it was in. This serves as a sort of extended undo if anything goes wrong. In the case of following a tutorial like this these incremental saves can provide a way back if the reader ever goes astray and doesn't realize it at first, as opposed to having to start all over, avoiding frustration.
Select File | Save As… or press F2.
With the pointer over the file list press + on the number pad to add a number to the file name.
Click on Save As Blender File to create the new file.