Boosts are a form of power-ups that can be applied to buildings and dragons. They can be synthesized at the Forge.
Some boosts can be used when you are defending a teammate (if you're in a clan). The boosts are
-Defensive shield (Shields buildings)
-Defensive Hammer (Heals buildings)
-Defensive Gunpowder (Symbol is a sword, (I'm pretty sure) it increases damage of the buildings it is applied to)
There are also other boosts that can defend your 'city' and others that can power up your dragon. Some of the boosts you will see also come with dragons sometimes, like for Hext, there is Havoc-deal more damage (basically) but Havoc can also be made into a boost (using the Forge)