In summary, the Buddha is teaching about tolerance, which is also one of the six paramitas in Mahayana Buddhism, and explains it this way:
The Buddha: “Suppose that a man were to come along carrying a burning grass torch and saying, ‘With this burning grass torch I will heat up the river Ganges and make it boil.’ Now, what do you think — would he, with that burning grass torch, heat up the river Ganges and make it boil?”
The Monks: “No, lord. Why is that? Because the river Ganges is deep and enormous. It’s not easy to heat it up and make it boil with a burning grass torch. The man would reap only a share of weariness and disappointment.”
The Buddha: “In the same way, monks, there are these five aspects of speech by which others may address you: timely or untimely, true or false, affectionate or harsh, beneficial or unbeneficial, with a mind of good-will or with inner hate. Others may address you in a timely way or an untimely way. They may address you with what is true or what is false. They may address you in an affectionate way or a harsh way. They may address you in a beneficial way or an unbeneficial way. They may address you with a mind of good-will or with inner hate. In any event, you should train yourselves: ‘Our minds will be unaffected and we will say no evil words. We will remain sympathetic to that person’s welfare, with a mind of good will, and with no inner hate. We will keep pervading him with an awareness imbued with good will and, beginning with him, we will keep pervading the all-encompassing world with an awareness imbued with good will equal to the river Ganges — abundant, expansive, immeasurable, free from hostility, free from ill will.’ That’s how you should train yourselves.
I thought this simile of the River Ganges to be very clever. A river cannot be boiled easily, especially by a small fire, so one should train their mind the same way and maintain a sense of good will at all times.
There is a similar story to this in the Lotus Sutra about the Bodhisattva Never Disparaging that reminds me of this sutra as well.
Namu Shaka Nyorai