Turbo in poker refers to a tournament where the levels are shorter, and the blinds go up more quickly than in a regular game.
For example, a regular MTT structure usually has 10-minute blinds when played online, and a turbo tournament might have just 5-minute intervals before blinds increase.
This leads to quite different game dynamics since players get to play fewer hands, and most of the action happens in a relatively short-stack environment. This makes the preflop strategy even more essential since you have little room to maneuver postflop and run big plays.
As a result, the variance is significantly higher in turbo tournaments, making them very attractive to recreational players. The events themselves are also shorter, so it might be a good option if you do not want to invest 8 hours playing a big regular event.