A bird that flies off the earth and lands on an anthill is still on the ground.

An anthill is a small mound of dirt. The bird does fly. It leaves the earth, it climbs, and it lands somewhere higher than where it started. It has done everything a bird does. And it is standing on the same ground it took off from.

The proverb does not say the bird nearly made it. It says the bird is still on the ground. It gives the flight no credit at all, because the only thing it measures is where the bird ended up.

So this is about a small gain that feels like a large one. A first raise, a better title, a move to the city. The gain is real and worth having, and it is still not the same as arriving. The question the proverb asks is how far you actually got, not how hard the climb felt.