The bottom timeline shows the time in the actual video.
Say, for example, you place a position keyframe on the bottom timeline at 00:00, then in your video at 00:00 (at the start of your video) the camera position will match with the one you had when you set the keyframe.
Similarly, when you place a time keyframe on the bottom timeline, the current time inside the replay will be stored in that keyframe.
So when, for example, the current time in your replay is 4:20 and you place a time keyframe in the bottom timeline at 00:00, then your video will start with the first 4m20s of the replay skipped.