https://www.khanacademy.org/math/geometry/hs-geo-foundations/hs-geo-angles/v/measuring-angles-in-degrees

This video is now shown first in the section on angles, but it looks like there was originally other content that comes before this that presumably introduces angles to us.

Also, in the last video from the previous section (Geometric definitions examples) and the practice session that comes after that (Practice: Geometric definitions) we are already supposed to be familiar with angles.

So I suppose originally there was a video introducing angles before that video and practice session.

Edit: I now see the missing content if I go to 

https://www.khanacademy.org/math/geometry-home/geometry-angles/geometry-angle-intro/v/angle-basics