The win streak event offers rewards for successfully completing levels in a row, without failing any levels. The longer the win streak, the better the rewards are.
Beat a level to complete the first stage of the win streak, and start the next level with a free booster. Pass the next level as well, and you'll start the following level with two boosters. Finally, beat a third level to complete the last stage of the win streak, and start the next level with three boosters. If you continue your win streak beyond three levels, you'll continue to start each level with three boosters. However, if at any point you fail a level, you will lose the win streak and have to start over from the beginning!