glossary

No mystery really.

Low end refers to the sail's ability to pull at the bottom end of its range. You can generate power either by increasing sail area or by increasing foil depth etc. Crudely a bagged out sail with a tight leech will give you more pull in a given wind speed but as it speeds up the drag will increase faster than the lift.

High end refers to the sail's ability to keep control and keep accelarating at the limit of its range.

Its easy to compare. Take a sail, put on less than minimum downhaul, keep the leech tight with no twist and have no outhaul tension. You've now got a sail with low end, should get you planing early but will be heavy and a handful to control once planing.
Now take the same sail, add on up to 6cm more downhaul, leech should be loose to 3rd batten, outhaul should be positive such that the foil looks almost flat on the beach. You've now got a sail that will not plane as early but will remain alot smoother in way higher wind speed than the first way of rigging.