Basics
This is how many views your video got from people clicking on your video from outside of YouTube.
Learn
Defined literally as "any link to a YouTube video video that does NOT come from within YouTube itself." EX: a bunch of people clicking on your video from twitter.
Gotchas
These views don't count towards CTR. You can get 1000 external views, and have a CTR of 0. These views don't come from "impressions", so they are ignored by YouTube when calculating CTR.