Explain Like I'm Five

Let's split the question up. First let's talk about carbon dating.

Carbon is an atom and we know an atom consists of protons, electrons and neutrons. For carbon that is six protons and usually the same amount of electrons. This is always the case, otherwise it's not carbon. The interesting thing is the number of neutrons because they can differ. If we have two atoms with a different number of neutrons we call the isotopes. The most common istope of carbon is C-12. This means it has 12 particles in its core. We know we have 6 protons, so we have 6 neutrons.

There is a rare form of carbon that is C-14, which has 8 neutrons. This form is not stable and breaks apart after a while and forms nitrogen. The time it takes for half of the carbon to break apart, or decay as we call it, is called half-time and for C-14 it's about 5000 years. That means every 5000 years the amount of C-14 halves. As long as an organism is alive it will have a pretty constant amount of C-14 in its body. As soon as it dies it can't consume more of it and then the clock ticks. We can measure how much of it is in the sample and with that we can calculate how old it is. This only rally works on a time range between 300 and 60.000 years. Shorter and not enough of the C-14 has decayed and longer and the so much has decayed that we can't measure it anymore.

For longer times we have other methods but I am unfortunately not too versed with too. Some of them also use the decay of elements but others that have longer half times. So they probably used one of these techniques to date your example.