The price of a barrel or the price for the consumer ??
The price per barrel, which usually translates to cheaper petrol prices at the pump for consumers. Happens through a few avenues.
1. Speculators fear American self-sufficiency on oil, and seeing as America is a big market for oil, the speculators get bearish (essentially they believe Oil as a stock/commodity will fall in prices). Oil prices fall, petrol prices fall as well. For countries dependent on oil for revenue i.e. Nigeria, Venezuela, Norway, Mexico, Russia, UAE etc This does a number on their balance sheets. Some will cope better than other i.e. Canada, Norway, UAE etc due to social net policies, sovereign wealth funds (essentially like a state-owned rainy day account) and diversified economies. Some will falter i.e. Nigeria, Venezuela etc.
2. As the US looks to enter the oil market, OPEC, which is largely controlled by Saudi Arabia and the middle Eastern oil producing states, will flood the market with that "cheap work" to paraphrase the great Gucci La Flare. Cheap oil means little to no margins for the US shale oil industry, so it comes to a halt. However, flooding the market with cheap oil is only a temporary fix, especially if the Trump administration subsidizes the shale industry.