Your answer to your question is correct. Isotopes of different compounds or element found in the rock are compared.The radioactive decay is related to half-life. Let us take in to a very common example into explaining how this works. You have a whole pie at your fridge that only you can eat since your other family members don't want to. On the first day, you eat the half of it. The second day, you eat the half of what's left. On the third day, you eat again the half of what's left. This continues until you finish it or you got sick over eating it. When your mom comes to look at the fridge and sees the pie is 1/8th already of what used to be a whole, considering that she know that you ate it half by half, then she will know that it is already 3 days old.
