A "good" trigger has a crisp and repeatable release with almost no pre-release movement (creep) and almost no after release movement (backlash). The first stage movement of a two stage trigger does not count as creep.
A "bad" trigger is pretty much the opposite.
Shoot a cheap air rifle or a cheap .22 and you'll experience the opposite of a good trigger.
The trigger on my 1885 Winchester single shot .22 has gotten noticably better with break-in, or maybe I'm just getting used to it.