The Refinery produces a steady stream of credits for your base.
The Refinery receives production bonuses from being adjacent to Tiberium and Power Plants. It receives a set bonus (based on refinery level) for being next to at least one Power Plant, and another set bonus (based on Power Plant level) per Power Plant. It also receives a bonus (based on Refinery level) for every adjacent Tiberium.
Thus, a Refinery's total output will be: Base amount (refinery level) + Bonus for being next to a power plant (receive only once, based on refinery level) + Bonus per power plant (receive once per plant, based on plant level) + Bonus per Tiberium (receive once per Tiberium, based on refinery level).
Like Research Points, Credits are pooled and shared across all bases allowing Research to be share across your entire empire without need to move them from base to base.
When on the offensive, the Refinery will contain cash, Tiberium, and Crystals, all for the taking. Destroy it, and claim them all!