Boost increases the number of event tickets you win when you play the event! How do you earn Boost?
Grab Boost Items
These are special items that will increase the number of event tickets you receive when you play the event. Though grab boost items are usually clothing items, you don't have to actually equip them to receive their benefits. 
To view Event Grabs, you can visit the event room and spin the grabs from the bots as usual, or you can spin directly from within the Event Menu > Event Shop > Grabs. 
Lucky Token Boost
You can spend your Lucky Tokens to directly upgrade your Boost. To do so, simply go to Event Menu > Boost > Lucky Tokens. You'll see a long list of Lucky Token Boost levels.
Important Things to Remember
Your Boost is calculated by taking 20 of your highest boosting Gold Grab items, and 4 of your highest boosting Lucky Token Grab items.

Account-bound items spun with Promo Tokens do not contribute to your Boost.