1. It meant that they are flat, unlike 3-dimensional objects: you can place them on a table and one of their sides will be a like. This also means that they cannot show depth very well.
2: It means that it can create an illussion or an abstraction, that we can consider esthetic. For example, while we enjoy drawings, which are 2-dimensional, it could be that we would consider their 3d-version scary
3: the limitation is that they cannot show depth. For example, an orange appears as a circle, not a sphrere: so a part of the information is missing.
