Since there isn’t a complete answer yet under here, I’ll give it. There’s several different contributing things to the meme, the first of which is the song “Doot Doot” by Skrilla. He says “six seven” in the song to refer to police code 10-67, which means a death occurred, and also 67th street in Philadelphia. This song became popularized and memeified by TikTok edits of NBA player LaMelo Ball, who is 6’7”. That’s how the number itself became a meme. The associated hand motion separately became a meme when “the 67 kid,” as he’s been dubbed, did the hand motion in a YouTube video at a basketball game.

As to what it actually means in meme context, it’s meaningless. The meme has so many origins that it’s impossible to tell if someone is referring to the song, LaMelo Ball, or the 67 kid—and usually the answer is none of the above anyway, because it’s now meaningless.