What does it mean when a sketch is over dimensioned?

It means that a feature is dimensioned more than once.

For example, suppose you have a drawing of a rectangle, and the width is dimensioned twice. That's an example of over-dimensioning.

Of course, that's a rather obvious example. Sometimes it's more subtle. Suppose the rectangle has the width and height dimensioned, but also the diagonal. This is also an example of over-dimensioning. If the width and height are dimensioned, then you don't need to dimension the diagonal; that's already fixed by Pythagorean theorem.