The Power Crystals are used to level up your meter and unlock a new weapon or upgrade during your mission.

Your level bar will grow as you collect more Power Crystals, and once the level bar is maxed out, you will be prompted to choose a new weapon or upgrade your current weapon based on your level.

Each Power Crystal is worth a different amount of power:

Keep in mind, once you choose a weapon, you cannot undo it, but you can unlock more weapons and upgrades as you advance to more sectors!