What is the meaning of the medical term "first line of treatment"

The meaning is exactly as it sounds and that is that there are first things to do like medications or therapies to begin with. If these are not effective then you go on from there.

This refers to the treatment which is preferred before any other because it works the best and is safe, generally as shown in medical studies (clinical trials). Over time, it may also have been shown to work the best during use.