More specifically to the Megasquirt it's "load". The best way to translate that is "percent manifold pressure relative to atmospheric". That effectively normalizes the table to operate in different altitudes. So 100% means the manifold pressure is the same as atmospheric pressure, or WOT. 50% would mean half of atmospheric pressure, or 0.5 bar/50 kPA/~15 inHg. Since atmospheric pressure is really close to 100 kPa (103 nominally), you can almost just read it as straight pressure/vacuum though.
Easiest way to interpret the table is left to right is increasing rpm and bottom to top is similar to throttle opening closed to open. So bottom left is throttle closed low rpm (idle) and top right is WOT redline. In my case idle is more like far left around the middle instead of the bottom, but same idea.
