This term is a little pet peeve of mine, like "ballooning". People talk about these all the time without being precise about what they mean, and it leads to confusion. So good for you for asking what it means. I can't tell you, but I can tell you what makes it confusing.

It is often spoken of in conjunction with wind, as in "wind penetration". But wind is just moving air, and the ability to move forward against it relative to the ground has another term that IS precise. It's called "speed". People will say that a heavier plane has better "penetration".

I think the confusion is this: in level flight, a heavier plane, all else being equal, is NOT faster than a lighter plane. It's slightly slower since it must fly at a slightly higher angle of attack and create more "induced drag" (drag created by producing lift).

But if your airplanes get to coast downhill, which is what gliders do, then it is a different story, and that's where Rock's comments hit the mark: more weight means more speed because they are always going downhill relative to the air. They're like a soapbox derby racer ... the heavier, the faster.

For powered planes, I think what people mean is that if your plane is downwind, and you need to get it back, the heavier plane has an advantage as long as there is some height to spare, because giving it a little down elevator will bring a heavier plane back faster. But for level flight, the extra weight does NOT improve "penetration".

I think it's a term that should not be used, and I don't think you will see it in aerodynamic texts.

Jim