Your laywer is time-reversed. Read the help.
All layers have a starting point, even stills and solids. Let's call the starting point Frame 0.
If you trim the starting point backward in time, you reveal frames that occur BEFORE Frame 0, effectively frame -1, -2 etc.
The line shows segments of the layer that occur before the original starting frame of the layer. So at the point where the line stops, your original Frame 0 occurs.