Decay Constant: This is the probability per unit time that a nucleus will decay. It's like how likely each popcorn kernel is to pop in our analogy.
Radioactive Decay: This refers to the process by which an unstable atomic nucleus loses energy by radiation. In our analogy, this would be like the popcorn kernel popping.
Isotope: Isotopes are versions of an element with different numbers of neutrons. Think about having bags of different brands of popcorn; they're all still popcorn (or still the same element), but they might pop at slightly different rates (like isotopes can have different half-lives).