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:

Self Control - 50%
Ballast - 17%
Warrior - 17%
Downforce - 6%
Instinct - 6%
Hex - 4%

You can watch up to 3 of these ads every 24 hours.