Cambium is like the tree's skin. It's the part of the tree that is just under the bark. The cambium helps the tree grow wider by making new cells. It's kind of like how you grow taller by making new bones in your body.
The cambium is really important because it also helps transport water and nutrients from the roots to the leaves. Just like how your blood vessels help transport food and oxygen to your body.
So, when a tree gets bigger, it's because the cambium is making new cells on the outside of the tree. This is why the tree gets wider and the bark gets thicker. Pretty cool, huh?