A bitcoin transaction to be considered confirmed requires usually 3 confirmations on the bitcoin network.
Every bitcoin transaction goes into what's called the memory pool before it can be confirmed by miners.
When there's a dramatic spike in transaction activity, the memory pool can become congested because many transactions are waiting to be included in the next block.
In that case bitcoin users across the network may notice their transactions sit as unconfirmed or pending for a long period of time. In most cases, these pending transactions will eventually confirm. It may just take longer time, in some cases depending on the fee paid for the transaction it may take even weeks until confirmed.
