Both are futures contracts, the difference is that the former will change. When May is the main force, it is the May contract. When the May contract ends, it will be replaced by the June contract. By analogy, 2006 refers to the contract in June 2020, and the crude oil futures contract is continuous every month. You need wind or other software to check the contract prices in different months. Generally, contracts within one year can be bought, provided that you have a corresponding futures account.