A polar front low is a low pressure system that's associated with the polar front. Polar front is the prevailing front (the red line in the picture) between the polar air cell and the Ferrell cell.
Sometimes a local disturbance in the polar front forms. How and why exactly these disturbances begin to form, is way above my education, but I don't think anyone else knows either (I read an article about that a few years ago, and basicly it said "nobody knows" spanned over 8 pages). There are several contributing factors, including the North Atlantic Oscillation, air pressures, winds etc.
Simplified though, "a dent" in the polar front forms and begins to rotate and move along the polar front. This local low pressure system is called the polar front low.
A cold and warm front form adjacent to the low pressure center. Both fronts have their own movement direction and speed, and they rotate around the low pressure center.
The whole system moves in the direction of the isobars in the warm sector, and with the speed of about 80% of the geostrophic scale measured between the first and second isobars in the warm sector (the red area in the third picture).