Resolution is the smallest distance between two points at which they can still be seen as separate. A microscope with a higher resolution shows finer detail. Magnification only makes the image bigger. If the resolution is poor, a bigger image is just a bigger blur.
A light microscope has a resolution of about 0.2 µm, which is enough to see the nucleus, the cell membrane and the cytoplasm of a cell. It cannot show the detail of smaller structures such as ribosomes. A transmission electron microscope (TEM) uses a beam of electrons that passes through a very thin slice of specimen, instead of light. Electrons have a much shorter wavelength than light, so a TEM has a far higher resolution and a far higher maximum magnification. Its resolution is about a thousand times better than a light microscope.
The TEM let scientists see the internal detail of structures such as mitochondria, chloroplasts and ribosomes. This gave a much better understanding of how these sub-cellular structures work. Electron microscopes are large and expensive, and the specimen is dead because it is in a vacuum, so light microscopes are still used to view living cells.