Staining specimen with heavy metal salts (e.g. tungsten, molybdenum) allows you to see the specimen better with higher contrast when electron beam deflects off of your sample.
In transmission electron microscope electrons are transmitted through an ultrathin section of a specimen to form a digital image. It necessary to stain the specimen with heavy metal salts to create a contrast such that various cell sub structures are easily visible. For example, DNA is stained with heavy salts such that it is easily visible against the background.
