So if you start with a sample of 1 kg, after one half-life, you have 1/2 kg. Then after two half-lives, you have 1/4 kg, then after three you have 1/8 kg, and so forth. Radioactive decay happens at a rate of 50% of the sample per half-life, rather than, say, a constant 1 kg per hour. They don't use the whole life because the whole-life is infinite, and thus unhelpful.

Soooo in one sense, the rate of decay changes constantly, from 1/2 kg per hour to 1/4 kg per hour and so on, and in another sense, it's constant- each atom always has a 50% chance of decay per half-life.