In radioactive materials a parent atom decays into a daughter, such as uranium-238 decaying into lead-206. For some radioisotope dating methods you have to assume you know the amount of daughter atoms in the crystal, so you can tell how much is a result of radioactive decay (and thus get an age for the rock). Please answer this question: why are zircon crystals so useful for uranium/lead dating? If you don't remember from class, can you read this article.