That red line lets you know that there is a time or sample gap in your timeline. When you drag a clip to place it adjacent to another clip, Adobe® Premiere® Pro 1.5 displays a red snap-line if the clip is not perfectly aligned to the edge of the destination clip. If you place the clip while this snap-line is visible, the clips will exhibit a gap that could span a frame or several samples, depending on your view mode. Premiere Pro also displays this snap-line when you drag clips to sequence markers, sequence In and Out Points, and the Work Area In and Out points.