A button in the "Boosts" section of the Store or on your Boosts page allows you to watch an ad and earn a random Boost for free.
It will always be one of the 6 base Boosts, and the drop rates depend on their value:

Common (EZ Going, Blam) - 40%

Rare (Compound, Energi, SupaGloo) - 16%

Epic (Battlement) - 4%