You can send a transaction to transfer coins but to become "real and permanent" this transaction needs to be confirmed, i.e. mined into a block of the blockchain. As nowadays there's competion for space in a block, the transaction needs to have a sufficient transaction fee, so that miners will choose to put it into a block.

If the transaction fee of a transaction is too low it will either be purged out of a node's mempool or it will stay for some days in node's mempool until it gets dropped due to "overtime" in mempool (a default value is about 14days, but nodes are free to configure some other number of days).

When a transaction gets deleted from all mempools and no node or wallet does resubmit the transaction, it is as if the transaction never happened and coins stay in the originating wallet/addresses.