A market maker broker quotes its own prices and takes the other side of your trade rather than passing it to an external venue. Its revenue comes from the spread and from the net result of client positions.
That second source is the reason for the model's reputation, and the reason to understand it rather than avoid it. Market making is how retail-sized orders get filled at all in many instruments, and a regulated market maker with segregated funds and published execution statistics is a legitimate counterparty. What matters is disclosure: whether the model is stated plainly, whether execution quality is reported, and whether the regulator behind it has a record of enforcing on either point.
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Guide
How a Broker Holds Your Money
Segregated accounts, regulatory capital and compensation schemes are three different protections that get described in one sentence. What each covers, what it does not, and how to check which apply to you.