Associations are not stable

Can you please explain "not stable" in other words?

And that is something that I think is stress, the feeling that we have to learn more than we really can. And in addition to that, it has been proven that when we learn something in a natural way, after one day, about 70% of it is not accessible anymore. Why is that the case? Because the brain is very good at creating associations, but those are not stable. After one day, many of those are lost, and you only got the main points of what you wanted to remember...