Carbon 14 is not as useful for fossils as for 'antiques'. It only works with organic materials, like wood or plants or animal bones. Fossils are just rock.
Anyway, carbon 14 is an unstable isotope of carbon. It decays with a half-life of about 5700 years. A living plant, like a tree, absorbs some carbon 14 in the air as long as it lives, but when it dies it stops absorbing it. From that point on the carbon 14 in the tree breaks down. So by measuring the amount left, you can estimate how long ago the tree or plant or animal died.
It's most useful back to about 50,000 years, which is another reason it's not useful for fossils. Fossils are much older than that.