A “dynamic” warm up simply means big movement for large muscle groups. Dynamic warm ups are performed in order to allow the oxygenated blood to flow to the working muscles, slowly raise heart rate, prepare the muscles for the work ahead, decrease the possibility of injuries & help us to mentally prepare for the workout to follow. I was always taught that dynamic warm ups are part of the “bell curve” which indicates that one should slowly prepare the body for the target training zone to follow. After the said amount of time in this target training zone, one begins to perform the cool down which allows the body to return to the pre-exercise state.
These warm ups can be choreographed moves to include movements that will be performed in the workout itself or simply sport/activity as Martin stated above.