Saudi and Russia got into an oil price war against each other. They flood the market with oil which drops the price of oil to see who can hold out the longest. Consequently, developing or competing oil producers (like those in the US) can't make any money when the prices are so low. Trump had the US purchase the low cost oil in order to stock up on our strategic reserve oil supply (National Defense), but still needs prices to be higher in order to allow US fracker/oil producers to be solvent. He negotiated the deal to have Saudi and Russian oil production limited (by approximately 20M barrels per tweet). Low supply and high demand should increase the prices.