Resources are sources or supplies from which benefit is produced. In Command & Conquer games, it refers to anything that is collected for credits to fund an army. This often involves processing it at a refinery and storing it in a silo.
In the Tiberium universe, the most common resource is Tiberium, with its green and blue strains. The exceptions are Tiberium Alliances, which introduced power and command points, and Tiberian Twilight, which replaced resources with a point system.
In the Generals universe, supplies are the main resource, with oil having the same role as in the Red Alert universe. Oil was used as a second resource in the cancelled Generals 2 until August 2013, when the original oil mechanic was restored.