Market Makers are basically the financial system's buffers: they exist to make sure that you can always buy or sell a given security/commodity. They are the people who make buying or selling something _immediately_ possible, rather than having to wait for someone who needs what you're selling (or is selling what you're buying) to come along.

This makes trades move quickly, which speeds up price discovery. This is good for market health.

Market makers make their money on the difference between the buy-sell spread. Of course, this is a bet: if the market moves against them, they can lose money on this. Often a market has one or more officially designated market makers who are paid to _always_ have a certain number of both buy and sell offers on the market to ensure that there is always liquidity. The payments help cover their risk

(These payments are often also materialized in the form of a "takers fee", i.e. a surcharge you pay whenever you place an order into the market which can be immediately filled)

TL;DR: They're a warehouse which will always buy and sell some product, except when talking futures we're talking wagons full of nickel 1 month from now and so the warehouse doesn't physically exist