The world and things in the world exist by repetition and the law of cause and effect.
Take rice farming as an example. Rice seeds sprout and grow, rooting in rice fields, and growing fruit. The fruit is again used as rice seed and repeats the same process. During this iteration, rice undergoes changes based on the quality of rice seed itself and the condition of land and environment.
In principle, humans are the same as rice seed. Like rice seed, a soul undergoes changes based on the law of cause and effect during the course of repetition.