Our understanding of cells has developed as microscopes have improved. The first microscopes were light microscopes. They use light and glass lenses to magnify a specimen, and they let scientists see cells and some larger sub-cellular structures such as the nucleus.
The electron microscope was developed later. It uses a beam of electrons instead of light. Two ideas matter here. Magnification is how many times bigger the image is than the real object. Resolution (resolving power) is the ability to show two points that are close together as separate points, so a high resolution gives a sharper, more detailed image.
An electron microscope has a much higher magnification and a much higher resolving power than a light microscope. This means it can be used to study cells in much finer detail. It has allowed biologists to see and understand many more sub-cellular structures, such as ribosomes and the internal structure of mitochondria and chloroplasts, which a light microscope cannot show clearly. This helped them to link the structure of these parts to their functions.