If a radioactive isotope is said to have a half-life of 5,000 years that means after 5,000 years exactly half of it will have decayed from the parent isotope into the daughter isotopes.
Then after another 5,000 years half of the remaining parent isotope will have decayed.
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.
Carbon-14, the radioactive isotope of carbon used in carbon dating has a half-life of 5730 years, so it decays too fast.
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.
Rocks are always changing in what is called the rock cycle. Take a look at this example of the rock cycle and how rocks can change from igneous to sedimentary and to metamorphic over time.
Who would have known that a rock would be so interesting?