In oil trading, it is important to know about the US crude oil inventories reports since they reflect the demands of crude oil which will directly impact the crude oil price.
Basically, there are two reports every week, one is by American Petroleum Institute (API), another is by Energy Information Administration (EIA). They measure the changes in the number of barrels of crude oil in the storage. For further reading on simple explanation of the differences between these two reports, you may visit Investopedia (EIA vs API Announcement).
In simple words,
Increase more than expected — lower demand — bearish for oil price
Increase less than expected — higher demand — bullish for oil price
Decrease more than expected — higher demand — bullish for oil price
Decrease less than expected — lower demand — bearish for oil price
One more thing to take note is that EIA report is one day after API report, and EIA report is a more stronger demand indicator than API report. Normally they would have the same direction.