Before watching the video, try to guess what the term 빠른 년생 means (unless you already know). Got it? Okay, let's continue.
This is a concept not well explained in textbooks (even my own) because it's an older system no longer in use. However despite it being retired (and technically no longer even legal), it's still a term that's used in Korea by Koreans - especially older Koreans. You might encounter this term used if you spend time living in Korea, and if you're a Korean then you'll have to be able to use it yourself.
While this system caused a lot of problems in Korea and is now gone, hopefully you'll be able to better understand what it was and why they got rid of it.