Networks and deposits
Mempool
The queue of transactions broadcast but not yet included in a block.
A broadcast transaction waits in the mempool until a block producer selects it. The selection criterion is economic: transactions offering the highest fee per unit of size go first.
That is why a send at the lowest fee tier can wait hours when the network is busy. It is not lost, but it often overshoots a rate lock window. On Bitcoin, the RBF mechanism lets you raise the fee on an already broadcast transaction.