The phrase means that the more more you have been exposed to, the more you have knowledge of. The phrases isn't talking only about traditional school learning, but it broadens the idea of education to encompass all of a person's experiences as having a contribution to the totality of education. For example, if you travel to a foreign country you would "see" new and different foods, language, culture, experiences. All of these will contribute to what you "know" about that place, and the world in general.
Every time you see (do) something you new, you are increasing the store of things that shape your understanding of the world. These experiences could be big or small: travel, a job, college, a movie, a book, anything! It all adds up!
I'd say it means that you learn /understand (know) more as you experience (see) more. Seeing things helps you to put things into perspective, it makes you more able to see things for what they really are.