Win probability is a percentage on each deal stage that indicates what is the probability of winning the deal. It allows you to prioritize deals in a stage.
To change the win probability of closing a deal at a stage, go to Settings >> Deal settings, click on edit default pipeline, click on edit deal stage, click Win probability dropdown menu and select an option. To create a custom probability type the percentage and add it.
Note: Ideally the deal pipeline should have both Closed Won and Closed Lost stage for accurate reporting.