Something available to be drawn on. Calling a thing a resource already frames it by its usefulness to someone.
Old French resourdre/ressource, from Latin resurgere (to rise again): re- (again) + surgere (to rise); a resource was first a rising-up-again, a reserve that could be drawn on to recover, before it meant simply anything available to be used.
Something available to be drawn on. The root's rising-again is a fair frame: calling a thing a resource already treats it as something kept in reserve for whoever needs it to rise.