Simply put, toe and heel are the areas of the solder joint that extend from the front and rear of the lead/pad. Think of a solder pad on your PCB as a piece of paper, and think of your hand as one of the leads coming off an IC package. If you place your hand in the middle of the piece of paper, the "toe" and the "heel" are the distance between the tip of your fingers, and the palm of your hand, to the edges of the paper. That's the extra solder in the joint that extends farther than the actual lead/pad on the IC itself.
Here, they are recommending a specific amount of toe because it will provide mechanical reinforcement for the joint. Since there is no room for a suitable heel, given that this is a castellated package that will sit pretty much flush against the main PCB, the only way to add more solder to the joint is to expand outwards, and thus they recommend to give it some extra toe.
To comply with their recommendations, you'll have to turn off the default stop layer and cream layer for the pads, and draw them by hand. This is as easy as going to the properties for the pads to turn both of those features off, and using the rectangle tool, selected to the tStop and tCream layer, to draw the features.
Based of the picture, it sounds like they recommended a tStop that matches the recommended pad exactly, and a tCream layer that matches the pad exactly except for the edge opposite the package, which would extend out 4 mils further.
Best Answer
The TStop layer is for solder mask, the thin coating that prevents copper on the board from being exposed except for the areas where desired.
The TStop layer leaves apertures slightly larger than the pads so that the copper is still exposed in case of misalignment when producing the PCB.
However, extreme misalignment can cause copper to be exposed by the wrong aperture and you wind up with a situation where two nets are both easily shorted by solder in the same aperture.
From Printed Circuit Design & Fab
Some PCB software will let you change the amount of pullback of the TStop layer, but ideally you should have some so that minor misalignment doesn't hide some of the pad, especially with fine pitch pads.