Resolution is the ability to see two points that are close together as separate points. A light microscope can only show detail down to a certain size, so very small structures look blurred. This limit is why ribosomes and plasmids cannot be seen clearly with a light microscope.
An electron microscope uses a beam of electrons instead of light. This gives a much higher resolution and a much higher magnification. In a transmission electron microscope the electrons pass through a very thin slice of the specimen, and the image is formed from the electrons that get through.
Electron microscopes let scientists see the fine internal structure of mitochondria and chloroplasts, and also show ribosomes and plasmids. This detail helped scientists understand how these sub-cellular structures work, which light microscopes could never show. The specimens have to be prepared carefully and are not living when viewed.