Market making is the act of buying and selling an asset within a reasonably short period of time to lock-in small profits. These market-makers make money by buying and selling within the bid-ask spread.
Market makers serve an important role within the trading ecosystem. They provide liquidity so that traders can sell their shares or contracts and get matched immediately. They then house the risk that the price of the asset will fall on their balance sheet while they attempt to offload it onto a different customer. As a result, they are essential to the functioning of a highly liquid market. Many market-makers in the modern day use algorithmic trading strategies in order to execute millions of trades–they may make a tiny amount of money on each trade but in aggregate they can make a large amount of profit.