When demand in the mempool is high, it may take a while for transactions to confirm. If a Bitcoin user wants their transaction to confirm quicker, they can tack a bit more money (bitcoin) onto their transaction to make it more lucrative for miners to mine. This is called the mining fee. It is not required to use a mining fee to get a transaction into a block, but the more extra funds one adds to a transaction the more likely it will make the next block.
Mining fees: the toll road of Bitcoin
Transactions in the mempool are competing to get into the next block similar to how drivers are competing to get somewhere when they use the highway. In both instances, both the driver and the bitcoin user can pay extra to get to their destination faster. A Driver may decide to use paid access lane to get some where quicker while a bitcoin user might tack a mining fee on their transaction to get picked up quicker.