Radioactive decay is random. In a sample of unstable nuclei we cannot predict when any one nucleus will decay. It could be in the next second or in a million years, and nothing we do to the sample changes that.
With a very large number of nuclei, though, the overall pattern is predictable. The half-life of a source is the time it takes for the number of unstable nuclei in a sample to halve. The activity (the count rate from the source) also falls to half of its starting value in one half-life. For example, if a source has a half-life of 3 days and its activity is 200 counts per second now, in 3 days its activity will be 100 counts per second.
You can model this with dice. Each die stands for one nucleus, and a die that lands on a chosen number has decayed and is removed. You cannot predict which die will decay next, but with many dice the number left falls in a smooth, predictable pattern. Half-lives are also used in radioactive dating, where the amount of a radioactive isotope left in a sample shows how old it is.