Each futures contract has a letter associated with a month so that traders, brokerages and exchanges all communicate the exact contract that is being traded.
For example, if we want to trade Crude Oil for the contract month of December for the year 2022, then the correct tradable symbol would be "CLZ22". "CL" is the product code for Crude Oil ("C" being the first letter in Crude and "L" being the last letter in Oil), "Z" being the letter code for the month of December and "22" being the last two numbers of the year we want to trade.
One more example would be the S&P 500 E-Mini Futures contract. The market code is "ES" for the electronic mini futures contract or "e-mini" as it is commonly known as. The "E" represents the word Electronic and the "s" represents the word S&P 500.
Quick side note, the S&P 500 stands for Standard & Poors 500, a list of 500 US listed publicly traded companies. In this example, we would be trading from the futures market rather than the stock market. Either one can be traded.
So the symbol would be ES, then the month code and then the year code. Example "ESH21" would be the S&P 500 contract for the month expiring in March in the year 2021.
Below is a list that is used by the futures exchanges and will apply to all US (and most non-US) based futures brokerage firms: