Dimensions are just axes of existence.

One dimensional is a concept; it is an infinitely thin, but straight line of any length. Technically any single, perfectly straight line, in any direction, is one dimensional.

Two dimensional is easiest to explain as.. a perfectly flat object, for example sprites in video games (flat images that exist on the width/height axes, but not on depth). These can only exist virtually, as any object in real life is going to have some depth to it in order to exist at all.

Three dimensional, our plane of existence and everything in it, has width, heigt, and depth.

With 3D covering everything, things would get really wacky if you'd add a 4th dimension, and such objects also only exist conceptually/virtually.