ELI5: A fee is required for every Bitcoin transaction sent. The bigger the fee, the faster it will get confirmed in the network.
The person who sent you this transaction has probably put a fee that is lower than the normal. This means that you will have to wait until the transaction gets confirmed, which may take a few days depending on how much was the fee.
Which wallet are you using? Get your transaction id and search it on
Blockchain.info - you will be able to see how much was the fee in the bottom of the page. e.g: In the picture bellow, it was paid a fee of 6 satoshis/byte which is super lower than the average and it will end up taking a lot of days to get confirmed.