The TL;DR
When there is high demand in the network, the mempool may become congested due to many unconfirmed transactions. This typically results in longer confirmation times and higher transaction fees, causing your Bitcoin transaction to take longer than expected.
Diving Deeper
The mempool size always changes based on the rate at which new transactions are being created.
Transaction fees usually spike because network users are forced to compete to have their transactions included in a block by paying higher transaction fees.
Therefore, your Bitcoin transaction has not yet been confirmed because the transaction fee you are paying is not high enough for miners to prioritize it. Usually, this means your transaction will stay in the mempool for a while until it’s added to a block by a miner.
If the fee remains too low to be added to any block, you will eventually receive the BTC back that you tried to send, enabling you to try again with a higher transaction fee.