When you make a transaction, it first appears in the network of the specific cryptocurrency you are using as 'unconfirmed.' This means the transaction has been announced to the network but not included in the blockchain yet. It is still pending and waiting to be completed.
Before an unconfirmed transaction becomes confirmed, it waits in what's called the 'mempool' (a sort of 'holding bay' for unconfirmed transactions). Each computer (or 'node') that validates transactions has its own mempool and thus, there's no universal real-time mempool status. The transaction stays in this state of limbo until a miner adds it into a block.
Several factors can influence how long a transaction stays unconfirmed.
An unconfirmed transaction is not a guarantee that the transaction will be completed. The transaction can be rejected by the network if another wallet uses some or all of the same inputs (i.e., digital currency) for another transaction.
While an unconfirmed transaction status can be stressful, especially when large amounts of cryptocurrency are involved, remember that a delay in confirmation does not mean the funds are at risk of being lost. It just means it might take some time for the transaction to be added to the blockchain.