I recently flew commercially (Southwest on a 737) and there was freezing rain predicted from 11pm to 9am. My flight was at 5:30am. As a result I didn't think we would end up taking off, but we did make it out with "only" a 60 minute delay. I started doing some research which quickly led me to discussions of holdover times, type IV deicing fluid, etc. and shortly thereafter got in over my head. After boarding the aircraft, there was a visible layer of ice covering the wings and the freezing rain was increasing in intensity. They went around the aircraft spraying something on it that appeared to blast all the ice off the surfaces. That was followed up with a green fluid that my research suggests is Type 4 deicing fluid. We then taxied to the runway and took off. Can someone more familiar with the airline-world explain the process in a way that a VFR only private pilot can understand?