Radiometric dating works by determining the ratio of the number of isotopes of an element and the number of isotopes the element it turns into over time. Since the rate at which certain elements decay and turn into different elements is understood, scientists are able to calculate the age of substances.
During the formation of the Earth, radioactive elements became embedded in the minerals that make up the Earth's crust. Since many of the minerals seen on a daily basis contain elements that decay over time, scientists can determine the ages of certain types of rocks.
The elements embedded in the Earth's crust decay over time because they are very unstable. When these elements decay and turn into other elements, scientists measure the difference between the original element and its decayed form. Since the rates at which these elements are fixed, all that's left to do is find out where the element is in its decaying process.
The rate by which these unstable elements decay are generally expressed in terms of half-life. Half-life refers to time it takes for half of the element to decay. This method of dating elements in minerals is commonly used in fields such as geology, archeology and anthropology.