US Department of Energy - Strategic Petroleum Reserve
Don't know much about it, but as far as I can tell, it's simply an oil "bank" that the DOE can use to stabilize fuel supply and prices in times of emergency. Emergencies like Hurricanes, supply destabilization due to war or embargoes, etcetera.
Flooding the market with millions of barrels of "cheap" oil (ergo: bought when it was cheap then stocked) can lower prices. But not for very long.