“Subhuti, in a previous life I met Dipankara Buddha, I had made many offerings and been very virtuous. If someone is able to receive, study, and practice this Sutra in a later, more distant age, then the happiness and merit brought about by this virtuous act would be much greater than that which I brought about by my previous virtue. In fact, such happiness and merit is incomparable. I know this might be hard to believe. Subhuti, you should know that the meaning of this Sutra is beyond understanding.”
It’s said that Siddhartha Gautama, the one we call the Buddha, wasn’t the first Buddha. They say that he studied with previous Buddhas from other ages in prior lives. That’s what this section is referencing. There are many stories about the Buddha doing good things in previous lives, sometimes as a human and sometimes as an animal. These are called Jataka tales and bear some similarity to fables in the western world. They are stories designed to teach children lessons about things like kindness, paying attention, etc. The Buddha in this section is saying that the merit of studying the Diamond Sutra is greater than merit that he generated in his previous lives.