Tutorial Step 13: Set the Baseline
When you have a schedule set up and balanced properly, and the project is ready to begin, choose Set Baseline from the Project menu. This copies the schedule you created into the baseline schedule. The baseline schedule remains unchanged as you update the actual schedule; as the project goes on you can keep track of how closely it is following the original plan.
You can choose Split from the Baseline/Actual menu on the toolbar to compare the baseline schedule to the actual schedule. In this picture, the schedule has slipped a bit compared to the original plan.