what is a dimension?

So what is the difference between 2d and 3d mummy? And what does 1d look like? And how come we can see things in 4d like the new smurfs movie?
Such are the random questions of a 6 year old, and I struggled to find answers or examples for some of these!
Saw lots of teasels and dragonflies in a quarry walk though. Feel nicely worn out!