I watch a lot of Rocket League pros on youtube. Most of the time, I see their competitor ask for a "sign". What does that even mean?
They mean "sign" their Steam profile (or comment on it). It's like getting an autograph from someone you look up to. The competitors are probably fans of the YouTubers you watch and that is why they ask. Other places you may see someone ask for signature like this is on Twitch in a larger/famous streamers stream (often through a donation message).