Back Porch is the portion in each horizontal scan line of an analog video signal between the end of the horizontal sync pulse and the start of active video at the left side of the picture. It allows time for a CRT monitor to “retrace” its electron beam from the right side of the picture to the left as it uses that beam to draw the picture from top to bottom, and left to right. In analog composite color video systems the back porch includes a “burst” of sine wave used as a reference signal for the color content of the subsequent line.

Mentioned in:
Video Sync Separator, Sync Stripper, Sync Generator 315 Series