Mempool
The mempool is a waiting room for transactions. You send a transfer and it lands there first. A miner or validator then picks transactions from the pool and puts them into a block. Until it is in a block, your transfer is not confirmed.
Every node keeps its own mempool, so they can differ a little. When the network is busy a queue builds up, and transactions with higher fees tend to go first.
The network is congested and you sent a transfer with a minimal fee. It sits in the mempool longer than the rest. Open a block explorer, paste your hash and check the status. Many wallets let you resend with a higher fee; follow your own wallet instructions.
What not to do
Do not send the same amount again until you are sure the first transaction failed. Otherwise you can pay twice. Check the status in a block explorer first.