Sure. In short, demand stopped exceeding supply.
Developments in technology allowed US companies to access underground oil reserves that were previously irretrievable. The US began pumping lots of oil to take advantage of high oil prices. Soon, the US produced more oil than Saudi Arabia and Russia. With oil flooding the market, prices started to fall.
Historically, when oil prices fell, Saudi Arabia would cut production, along with the rest of OPEC (a collection of the big oil producing countries). But they didn't this time. Many think that Saudi Arabia didn't want to lose their market share and wanted to drive out the new US competition (OPEC can survive on lower prices of oil, the US and Russia can't).
Whilst supply was increasing, demand for oil was falling. China, one of the world's largest oil importers, faced a slowdown in growth and cut back on oil imports. Europe was also on the brink of a recession. Reduced demand led to an excess of oil and depressed prices further.
Thank you! That was so much clearer than what I was reading before.