Mean dimension is like counting how many different ways you can draw a shape on a piece of paper. When we say "mean dimension," we're talking about the average amount of space that shape fills up on the paper.
For example, if you draw a circle and fill up most of the paper with it, the mean dimension of that circle is high. But if you draw a tiny circle in the corner of the paper, the mean dimension is low because it's taking up less space on the paper.
Mathematicians use mean dimension to help them understand how much space different shapes take up in different situations. It's kind of like measuring how much space something takes up, but measuring it in a special way that helps us compare it to other things.