It's when a champion is calling out that they're going to solo the mid lane since there are 3 lanes and 5 players. The player in the middle needs to be able to move around quickly and gank well usually. They'll come to top / bottom to help do that extra oomph of damage to help finish people off in either lane in between guarding his/her own.
Almost always a carry so they can solo a lane and get better xp/gold (not shared) and by ganking will get more killshots.
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