I really want to know what is "past life" and what is the relationship between the two?

The so-called past life is the life of your previous generation, and this life is the life you are writing now. Religiously speaking, the past life has passed, and this life is a life of getting corresponding rewards and costs for what you did in the past life. Buddhism pays attention to the causal cycle. They think that if you didn't finish what you did in your last life, you should pay it off in your next life. Commitment to society, the past life is the past history. You have lost it, and you can't have it anymore. And this kind of life is what you should cherish now, because it has not become the history of the past, it is not a fantasy of the future, it is your most precious thing, at least you still have it now.