What is a maker-taker model?

Let's look at the following example:
Alice sell 1 BTC at $1000
Bob sell 1 BTC at $995
Charlie buy 1 BTC at $990
Dave buy 1 BTC at $985
If you add a buy order at $1000 for 2 BTC, you will immediately match the offers of Alice and Bob. You're a taker.
If you add a buy order at $993 it doesn't get matched and you're a maker.

Most of the crypto exchanges use the maker taker model to decide trading fee on the exchange.

Source: StackExcchange