There was quite a lot info about it, but I didn't quite catch what happens if I don't cancel it? It's stuck for a period of time and transaction either goes through or returns?
I have no idea what I did, but clearly something. Should I cancel it? If I cancel this then transaction just returns and I've paid some fees "for nothing"?
It means that your transaction fee is very low for miners to add it to ths blockchain. If you transaction supports RBF, you should go and increase your transaction fee with the fee slider by adjusting it to the right. You can use
https://mempool.space/ to check for the appropriate transaction fee to use.