What's a "Low Priority" Zone?
Hugh the Ducky says:

This is useful when you are heating areas you do not hang around in much, like a garage or workshop, so if they are not as warm as fast, that is not a big deal.
In the HUG Hydronics system you can make this setting selection by moving a jumper to pins on the control board marked "low priority" over heating zone 4. You can also change this setting just by removing the jumper from the pins.

What's a Jumper?
A jumper is a little black piece that connects two pins; connecting the circuit like an “on” switch. Jumpers are used on each piece of the control board to allow you to easily choose options by which pins you put the jumpers on.
The control board comes with jumpers already in their most standard configurations, and there are extras in the accessory bag.