If you don’t know what gas fee is, please take a look first this post. High fees are probably the biggest problem of the Ethereum network at the moment.
When does this happen? When there are a lot of transactions (people sending ETH or trading ERC-20 tokens). The explosive growth of DeFi in 2020, in particular, has significantly increased the number of operations performed every second.
However this issue should be fixed by the 2.0 upgrade which aims, among other improvements, to increase speed while reducing costs for users.