P = Predecessor S = Successor

This dependency states that you can start the successor after you start the predecessor. There is overlap between the two activities because you don't require 100% of the predecessor to be complete before starting the successor. Creating overlap will shorten the schedule, but it also increases risk. If something happens in the predecessor that impacts what you've already built in the successor, you'll end up redoing the successor work. The assumption with the SS dependency is moving both activities to start as soon as possible.