If you are talking about a compression fitting with a brass ferule, they sometimes stop leaking because minerals and crud in the house plumbing system will flow into the leaking area and eventually plug up the leak.
Very similar to how your own body stops the flow of blood from a small cut.
Blood platelets will fill up the leaking area in your blood vessel and form a clot.
If you have tightened the fitting properly and the leak is slowing down, just put a bowl or pan under it for a few days, it might stop dripping.
However, the leak might start again if you ever disturb that connection in any way.