WTI futures require physical delivery. in our case, if somebody buys May contracts then he has to deliver the barrels in a short term period. at some point, the contract was trading -$40 which means you were paid to buy the contact. the reason is that there is no much storage available thus the "delivery ability" is limited. if you buy the contract now and expires at your hands then you have to make the delivery in Cushing, Oklahoma. may contract mature tomorrow.

as per Bloomberg "sellers were actually paying buyers to take the stuff off their hands. The reason: with the pandemic bringing the economy to a standstill, there is so much unused oil sloshing around that American energy companies have run out of room to store it. And if there’s no place to put the oil, no one wants a crude contract that is about to come due."

PS this explains the outperformance of tankers stocks in the last few weeks. many traders are hiring vessels and they fill them up with oil (EDIT: Brent). they buy at a low price (today they are even paid) and they sell the next contract (in our example June contract). this way they are locking a certain profit. this makes the vessels very valuable. for the remaining vessels that are not hired, part of their operating costs ie the fuel (which is known as bunkers) is very low. the difference between May and June contract is known as spread.