"Aw, here it goes!" was 90s Nickelodeon sketch show Kenan & Kel’s version of "LIve, from New York, it’s Saturday Night!" What’s interesting here is all the weird little variations Kel does. Sometimes he dances before he says it, or he has someone else say it. It was their "Oh my God, they killed Kenny!" It was just a thing they had to say.
You can actually see Kel age in this video. Compare an “Aw, here it goes!” at the beginning of the video to one at the end. It’s not often you get to see someone age in a freaking supercut.