What is a Front Porch used for?
What is a Back Porch used for?
What does it mean by active video?
Both front porch and back porch as well as horizontal pulse and color burst are sync information in the horizontal blanking period, in the analog video.
The front porch it is an interval period between the end of picture information and start of horizontal pulse. The level of front porch is at pedestal (black reference) and the purpose is to set blanking level ("clear" of any signal level remains) before the horizontal pulse occurs. Duration it is very short, 1.5μs.
Back porch is the duration between end of horizontal pulse and start of the next line with video information. Lasts more than front porch around 4.7μs and the main purpose is to give the time to beam scanning for reverse direction (right to left) to start new line. In addition it sets the reference black level as well as sets the timing of color burst pulse.