What happens if I send a bitcoin transaction with a low miner fee?

If you send a bitcoin transaction with a fee that is too low, the transaction may take days or weeks to confirm, and sometimes it will not confirm at all, sending it back to the original wallet. 

A sufficient miner fee makes it more likely that your transaction will confirm in a short period of time. A great resource to check the current miner fees is https://mempool.space/. Here you will find what the current fee average is by checking the statistics of the latest block.