Market makers play an important role in increasing the accessibility and liquidity of cryptocurrencies to traders, investors and market participants around the world. Market making is an activity whereby a trader simultaneously provides liquidity to both buyers and sellers in a financial market. Liquidity is the degree to which an asset can be quickly bought or sold without notably affecting the stability of its price.

This article may explain about Market Makers in bit detail : https://blog.drhack.net/understanding-market-making-in-the-cryptocurrency/

In short market makers are the liquidity providers on a crypto exchange. Which allows other users of the exchange to trade.