Once people have reached a certain age, they have taken enough lumps over the years to realize that they should pay attention to what others say. They don’t necessarily have to agree with everything they hear, but they know it is a good idea to be open to learning something new and to possibly be proven wrong. Someone who does this is said to be humble. 
One can also be humble with respect to experiences. Learning from past mistakes (and successes!), either made by oneself or others, is also humility.
Humility is not an intrinsic quality. It is a state of activity. It is seeking feedback, not waiting for it to come.
Some people intend to practice humility, but they end up landing instead on false modesty. This comes from mimicking the demeanor of people who are greater without understanding the underlying principles that make these greater people appear the way that they do.
Humility is not superficial. In fact, I think one can be arrogant but still be humble. For example, I am rather arrogant about my humility because I harness it to such a degree in my day-to-day work that it is effectively weaponized.
 Humility may be used to mean “thinking lowly of oneself.” In this essay, I am using it in the sense used by Kant, which acknowledges our failings but also reaffirms our ability to discover truth.