Biological material is mainly composed of molecules containing atoms like carbon, hydrogen, oxygen, nitrogen, phosphor etc., which all have a low atomic weight and are therefore electron-transparent. In the electron microscope such untreated samples would yield a blurred image with low contrast. To increase contrast, samples have to be treated with salts of heavy metals which are able to scatter electrons.
See section Stains in Electron Microscopy for the most common contrasting reagents.