In TBB, an XP boost usually means you earn more experience for a limited time or set number of matches, so you level up faster. In the current forum/wiki info I found, it appears to be a boost that increases EXP gain by 100% during its active window, and it applies to matches rather than quest rewards.

What it does

  • Increases EXP gained while it is active.
  • Helps you level faster by speeding up progression.
  • Based on the info found, it only affects match rewards, not other sources like quests.

Extra detail

The boost also stacks in a specific way with VIP Boost, rather than just adding the percentages together in the simplest way. That means the final XP amount can be higher than a plain “2x” description suggests depending on what else is active.

Quick example

If you would normally get 100 EXP, the XP boost alone would raise that to 200 EXP, and with VIP Boost active the total can be even higher depending on the game’s stacking rules.

TL;DR: XP boost in TBB is basically a progression booster — it makes you earn more EXP faster, mainly from matches, so you level up sooner.