A single REC stands for 1 Megawatt-hour of renewable energy using an electricity grid, with the full suite of related attributes such as environmental benefits. RECs function as contractual or trading entities, which are tracked and matched to officially record your use of renewable electricity.
In the US, you can only claim the usage of grid-connected renewable electricity with RECs. RECs can also be utilized as compliance instruments for delivery and consumption-based state Renewable Portfolio Standards. Please also note that specific states have different regulations regarding the definition and use of RECs.