Knowledge, ultimately, is an understanding of the cause and effect relationships that govern our lives, the nature and role of each entity in relation to all. Knowledge allows us to describe, affect and predict the natural world. Without knowledge, values and dreams are impotent; even the best intentions can go very far astray.
We acquire knowledge in fragments that are unique to each individual, which is why we can be smarter together than any one of us can be alone. For groups as well as individuals, knowledge is partial, limited by experience and the structure of the human mind. This is why confidence must always be tempered by humility-- a recognition of fallibility.
The most serious barrier to knowledge is the failure to recognize our own ignorance, the unknown unknowns. To correct errors and fill gaps is the work of a lifetime--and more. For what we know, we owe a debt of gratitude to our ancestors and to the most curious among us.