OP phrased it weirdly but what they mean is that trees have all growth happen at the surface, not in the middle.
which is actually the opposite of how our skin grows, skin cells grow at the bottom of the skin and are pushed upwards by the new growth and mature as they go, until they reach the surface and die so that they can protect us and easily be shed to make space for new cells.
which is why you’ll see hollow trees being perfectly happy and healthy, whereas a human with an empty space underneath their skin is going to be uncomfortable at best.