It means that travelling is likely to open up your mind. Bit related to:

“seeing is believing”: you need to visit another place and see it in order to believe how things work there.
“Potemkin village”: you may end up deceived because people are showing you only things they want you to see.

This proverb is IMO less true today than it used to be, killed by globalised tourism. Fly over the earth, stay in a tourist resort, never learn anything about the local life and culture.