Transshipping is when your cargo changes vessels en route.  This can occur when a certain vessel does not go the entire range of your booked route.  For instance, if you book a container from, say, New York to Accra, Ghana, the vessel will likely stop in various ports along the way (e.g. Algeciras).  In some cases, your container will be removed from the original vessel and placed on a second vessel.  The amount of transshipping can vary based on carrier and the routing.  However, your cargo remains sealed in your container the entire time.