A market maker is a broker-dealer, who has the capital to facilitate large transactions between buyers and sellers. They tend to hold a ton of shares, so that they can help facilitate these large transactions.
In other words, they are the only party that is providing both the service and the good, so they can make the rules, AKA, determine the price... hence "making the market".
The 'Market Makers' goal is to make money, but not the way that you and I would want to make money. They do not buy and hold stocks, hoping that the stock goes up. They make money on what is called the "spread". Which is difference between the best bid price and the best offered price for that specific stock. I drew a shitty diagram below on what it looks like:
Since these 'Market Makers' have a ton of 1) capital and 2) inventory of stocks, they can manipulate the market by buying/selling/filling/not filling contracts and trades until they feel they are able to make the most money. As you continue to learn more and more about the stock market, you will be told to think like a "Market Maker" which is something that does not come very easily. Be cautious because they are out there to take money out of your pocket and put it into theirs.