The active time segment is the total range of frames that you can access using the time slider.
By default, the active time segment runs from frames 0 to 100, but you can set it to any range from the Time Configuration dialog. In addition, the active time segment can include negative frame numbers, so you can create keys before frame 0 and work in negative time.
You can change the active time segment whenever you want without affecting the keys you've already created. You might think of it as a window in time, specifying only that portion of your animation in which you want to work. Thus, if you have keys scattered over a range of 1000 frames, you can narrow your active time segment to work on only frames 200–300 without affecting the keys outside of the segment.