Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
It’s often much easier to date volcanic rocks than the fossils themselves or the sedimentary rocks they are found in.
So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks.
If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between 410 and 420 million years.
The same rock formation also contains a type of trilobite that was known to live 415 to 425 million years ago.
Radiocarbon decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food.