Radioactive elements change spontaneously to non-radioactive elements. For example, radioactive uranium changes, or decays, to non-radioactive lead. If you start with a certain amount of radioactive uranium (100% U) it takes a certain amount of time for one half of the uranium to become lead (50% U,50% Pb). Then it takes the same amount of time for that half to become one half lead (25% U, 75% Pb), and so on. This is called the half-life, or decay constant; it never changes inside rocks.

Different radioactive elements decay at different rates. Some radioactive isotopes have short half-lives, others have very long half-lives, such as millions or billions of years. This is where you need a fancy machine and laboratory to find out how old Earth is; it is done by actually measuring the very tiny amounts of radioactive elements and their decay products, called the parent and daughter, in rocks. If you know the amount of parent and daughter (such as uranium and lead) in a rock and the decay constant, you can calculate the age of the rock. By measuring parent-daughter ratios for various radioactive elements in many kinds of rocks, and meteorites, we know that Earth formed about 4 and 1/2 billion years ago and that the Sun formed shortly before then.