The Bitcoin network is secured by mining, which consumes electricity. That means there is a real-world cost to keep the miners running, which must be paid for somehow. Currently, the Bitcoin network mints new coins in order to pay miners, in the form of a fixed reward in every block. This has worked fine for the first decade, but since the mining reward is set to gradually decrease and eventually stop at 21 million coins, we’ll eventually have to find another way to pay for the network’s security.
People typically respond to this issue with two strategies: fees or inflation.