Through radioactive decay, a radioactive isotope (the parent atom) converts to a new form (the daughter atom).
An isotope's radioactive half-life is the amount of time it takes for half of the radioactive material in question to decay to a new form.
Consider parent material with 100 radioactive atoms.
After 1 time unit—meaning 1 half-life—half of the parent material has decayed to daughter material through the loss of subatomic particles.
After 2 half-lives, 25% of the parent material remains.
After 3 half-lives, 12.5% of the parent material remains, and so on.