I know what a dimension is but I don't understand the use here.

The theme is “dimension”, you can interpret that however you wish.

You could for example make a game about measurements of objects, or about alternative realities. Here is a list of possible meanings of dimensions: https://en.wikipedia.org/wiki/Dimension_(disambiguation)

Perhaps have a watch of this to help with ideas: https://www.youtube.com/watch?v=xe2X0WJgI-4