The YouTube video "Class 3 | Wet pipe sprinkler systems" by Irfan CFPS explains the operation of wet pipe sprinkler systems, the most common type of automatic sprinkler system used worldwide. The video discusses the three main types of sprinkler system piping arrangements: tree systems, loop systems, and grid systems. It emphasizes the importance of choosing the correct sprinkler system type based on the specific needs of the building or space being protected, such as temperature considerations. The video also outlines the components and functionality of a wet pipe sprinkler system, including how it automatically detects and extinguishes fires.