Flasks are used to completely refill energy. They are found under the General tab of your inventory if you have any. Sometimes players use the term 'flask' as a verb as shorthand for 'use a flask'.
There are 3 types:
Titan Energy Flask
Looks like a purple bottle with a skull on it. Instantly refills your Titan energy to a maximum of 3.
Many alliances coordinate whether anyone in the group ought to use a flask. Some alliances choose to save them for rare titans.
Raid Energy Flask
Looks like a blue bottle with crossed swords on it. Instantly refills your raid energy to a maximum of 6.
Most individuals use raid flasks when it will help fill wanted chests more quickly.
World Energy Flask
Looks like red bottle with a dragon's head on it. Instantly refills your world energy.