Bonuses are extra products offered to all buyers of a bundle if a specified sales goal is met. Once a bonus goal is reached, the bonus products will be added into the accounts of everyone who had purchased the bundle, with no requirements on the tier or products chosen at the time of purchase. Each goal is usually composed of one bonus product, but sometimes it may be composed of multiple products.
For unlocked bonuses, you don't have to do anything to select them when purchasing a new bundle. They are included automatically. For Build-A-Bundles, they do not count towards the number of products chosen.