Humility describes the concept that we should, in general, be less sure about what we know than intuition implies. It is closely related to epistemology.
The basic concept here is that humans are over-confident on average (Inside-view forecasting is a classic example) -- people are not only wrong, they are very confidently wrong. Consequently, it seems to be of benefit to assume that your assessment of confidence (how sure you are in a given theory) is overconfident in any given instance, and to plan accordingly.
The eighth virtue is humility. To be humble is to take specific actions in anticipation of your own errors. To confess your fallibility and then do nothing about it is not humble; it is boasting of your modesty. Who are most humble? Those who most skillfully prepare for the deepest and most catastrophic errors in their own beliefs and plans. Because this world contains many whose grasp of rationality is abysmal, beginning students of rationality win arguments and acquire an exaggerated view of their own abilities. But it is useless to be superior: Life is not graded on a curve. The best physicist in ancient Greece could not calculate the path of a falling apple. There is no guarantee that adequacy is possible given your hardest effort; therefore spare no thought for whether others are doing worse. If you compare yourself to others you will not see the biases that all humans share. To be human is to make ten thousand errors. No one in this world achieves perfection.
-- Eliezer Yudkowsky, The Twelve Virtues of Rationality
(And, just to prevent life becoming too easy, make sure not to become underconfident in the process of avoiding overconfidence!)
In LessWrong parlance, this should not be confused with "epistemic modesty" / "modest epistemology". While Eliezer lists "humility" as a virtue, he provides many arguments against modesty (most extensively, in the book Inadequate Equilibria; but also in many earlier sources.) Humility is the general idea that you should expect to be fallible. Modest Epistemology is specifically the view that, due to your own fallibility, you should rely heavily on outside-view. Modest epistemology says that you should trust average opinions more than your own opinion, even when you have strong arguments for your own views and against more typical views.
Related Sequences: Inadequate Equilibria
Related Tags: Calibration, Chesterton's Fence, Underconfidence, Modest Epistemology