Spark is a function in the game, which unlocks as soon as you have unlocked live events. You can power up your Transformers characters with it to earn better rewards!

This is a permanent feature, so you can keep adding sparks to a character to keep powering them up. Sparks you have used to promote bots stay on when the Tournament changes. Moreover, Sparks can be found in Steel and Gold crates, Spark runs, Token Exchanges, Daily Quests, and Special bundles.