A mistake that people often do is to think about the past, being upset about things that have already happened or things that cannot be undone. This is what the quote is about, and it might be a great advice actually.
First of all, "crying over spilt milk" refferes to an action that can happen to everyone by mistake. The point is to not worry about it because it is not such a big deal, just as some upsetting events in life. This proverb says that we shoud not worry about the things that already happend, and just forget about them, focusing on the future.
Second of all, it can really be an advice, because when we do a mistake we should not really criticize ourselves for doing it, but to get over it. Unfortunately, the mistakes done in the past can not be undone, so there is no point in worrying and crying so much about what we have done, because this won't resolve anything.
In conclusion, the quote tries to explain a very common thing among us, because every on of us happen to cry over something that we have done in the past. But is it really worth it ?
