What is meant by the half-life of a radioactive isotope?

The half-life of a radioactive isotope is the time that it takes for a radioactive isotope (meaning that it is unstable and will emit particles and energy) to decay to half its original amount. For example, you have 18 g of fluorine-18. If its half life is 40 days (which in reality, it probably isn't), then after 40 days, you'll only have 9 g of that element. The 9 g of fluorine-18 that was decayed in the forty days is now another element.