Carbon dating accurate not
So looking at things like rock and organic material that take carbon in when they're formed, if we check how much carbon 14 they have now, we can form a pretty good measure of how old they are.
(The further back, the less carbon 14 is found)Being able to make that calcination depends a lot on knowing how much carbon 14 was about when the material formed in the first place.
For radiocarbon dating specifically, lifeforms take up carbon from the atmosphere in both radioactive, and non-radioactive forms. Over time, the radioactive form will decay, while the non-radioactive does not.
So if you know the half-life (which we do), you can tell how much time has passed by how the ratio has changed from what would be expected in the atmosphere.
However, the carbon-14 already in your body starts to decay. Since we know more or less how much carbon-14 you started with, counting how much you have will tell us how long ago you died.
An example, with completely made-up numbers: Say that the standard for living things is that 10% of their carbon is carbon-14 (this is way too high, but it makes the numbers easier).
But carbon-14 also has natural fluctuations due to volcanoes or interaction of cosmic rays with nitrogen in the atmosphere.
If it has 2.5% carbon-14, it died around 11,460 years ago.
The more time that has passed, the less radioactive carbon to carbon.
The reliability can vary depending on the sample preparation, the testing time, the age of the sample (samples older than 50,000 years potentially can't be tested at all as too much radioactive carbon will have decayed to get a good measurement), atmospheric events, calibration, and so on.
Radiocarbon dating is a subset of radiometric dating.
These depend on tracking the decay of some radioactive substance at a known rate (the half-life of the substance) against some known start point.
Its reliability is not perfect, but most of the time isn't off by more than 3-5%.