Approximately once every ten minutes, the Bitcoin network issues one "miner" with a block reward of freshly minted bitcoins. When bitcoin launched in 2009, the block reward was 50 bitcoins. Every four years, the reward is cut in half. Currently the block reward is 12.5 BTC. In May 2020 it will go down to 6.25 BTC. (Here's an interesting page with live data about bitcoin and mining rewards.) The only other incentive besides block rewards are transaction fees miners require to add a transaction to a block.
So what happens when the mining reward becomes so small as to be inconsequential (it will reach zero in about 120 years, and the reward we be on the order of 0.00000001 BTC, or about 1/100 of a penny at the current exchange rate). In this video Heidi of Crypto Tips describes of the scenarios that could play out.