Tracer elements are radio isotopes such as sodium - 24, iodine-131.
In a funny way, we can call them as informers. When sodium -24 is injected into the blood, then its passage could be easily studied as they emit radiations in a smaller and safer amount. Thus, the rate of blood flow is ascertained and there by we can know about the functioning of the heart.
Same way, iodine-131 injected would be followed and the thyroid functioning can be studied as thyroid absorbs iodine.
Similar way, so many tracer elements such as phosphorous-32, given to the plants would give the details of the manure taken by the plants by what amount, at what season etc.