Shaping is a learning technique where successive approximation and a Positive Marker are used to teach behaviors and communicate concepts.
Successive approximation essentially means continually closer to the target behavior. So Shaping is a learning technique where you continually get closer and closer to the target behavior using a Positive Marker to tell the dog when she is right.
It sounds like a long hot mess, and improperly done, it can be, but dogs who understand Shaping are very good at figuring out what the handler is looking for. Skillful Shaping can quickly fix or avoid problems and teach seemingly impossible skills.