By default, the active time segment runs from frames 0 to 100, but you can set it to any range from the . In addition, the active time segment can include negative frame numbers, so you can create 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.