A win boost is an order for a fixed number of competitive wins rather than a target rank. You choose the playlist, the number of wins and whether you want to play along, and the booster keeps playing until exactly that many ranked matches have been won. Only wins count towards the order, so a lost match never reduces the number you paid for. It is the simplest way to buy a clear, measurable result without committing to a whole climb.
Every ranked win raises your matchmaking rating, usually called MMR, and every loss lowers it. The rank and the four divisions you see inside it are only a picture of where that rating stands, so a series of wins pushes you through the divisions and eventually into the next rank. How much each win is worth depends on the rating of the lobby, which means wins against stronger opponents move the bar further. A win boost turns that mechanic into a guaranteed number of steps forward.
The order can be placed for any competitive playlist the game offers. Duel is played one against one, Doubles two against two and Standard three against three, and each of them keeps its own rating and its own rank. The Extra Modes, such as Hoops, Rumble, Dropshot and Snow Day, are ranked separately as well. The booster only plays in the playlist named in your order, so the wins land exactly where you want them.
Players order wins for different reasons. Some want progress towards their season rewards, which depend on winning games at your rank, and are running out of time before the season ends. Others want to lift a poor win rate, finish a challenge that asks for victories, or simply get the momentum to cross into the next division. Whatever the reason, a win boost gives you a result you can count, match by match.