Checking the Include/Exclude Weekends toggle on the Milestone Plan or Gantt page of a Plan allows you to switch on or off the ability to schedule work on weekends as working days. If this toggle is set to Excluded and a Task or Milestone is moved/dragged or its start or end dates are changed the Task or Milestone will be moved appropriately and its start and/or end points shifted to non-weekend days. If the toggle button itself is clicked then all Tasks or Milestones will have their dates potentially modified if they are on or cross a weekend.
Note that the toggle action is not fully reversible, e.g. if you have weekends Included and have some Tasks starting on the weekend and then toggle to weekends Excluded, the start date will be moved to the Monday - but if you toggle back to weekends Included, the start of that Task will remain on a Monday (since the system cannot know/remember that it used to start on a Saturday)