This quote is telling us that sometimes, people don't realize how great they are and think less of themselves. At the same time, they might think too highly of things they don't actually possess or can't do.
Imagine you have a special toy, but you don't realize how amazing it is, so you don't take care of it. On the other hand, you might want a toy that someone else has, even though it's not as special as your own.
In simple words, the quote means that people often don't see how wonderful they are and focus too much on things they don't have. It's like not appreciating your favorite toy and wanting someone else's less special toy instead.