Imagine walking through this insane rain storm?
Torrential rain or a torrential downpour, is any amount of rain that is considered especially heavy. While the word may sound like another severe weather type -- tornadoes -- this isn't where the name comes from. A "torrent", rather, is a sudden, violent outpouring of something (in this case, rain).
Although most of us just love sudden and unexpected summer rain, this is a very extreme version that nobody would find enjoyable.