To upload a video to YouTube or a similiar video-sharing website using a different account then the one you usually use. This is done to avoid being responsible for the embarassing/incriminating content in the video
One: "Dude, look at this totally gross and disturbing animation I made!"
Two: "Hahaha, that's hilarious."
One: "Yeah I'm uploading it to YouTube."
Two: "Dude, make sure you're alt'ing that shit. I don't wanna look sick in the head."