Celerium
Celerium is the premium currency in this game. It is very valuable because some things can only be bought with it like the very important Builders which allows you to construct or upgrade structures simultaneously. It can be used to boost your facilities be they resource production or unit production. If you don't plan on using real money however, you should choose what you spend it on. A good option is to focus on extra builders.
These can be obtained mainly from the Vault where you purchase them with real money, events, 5th reward in the Daily Log in, randomly from objects around your base you clear, daily PvP reward, Achievements, landing zone random reward, etc..