This is a wad of incidental dialogue from one of the personality spheres from the end of Portal 2. You may want to not watch it if you haven’t finished the game yet, but there’s only half of a plot spoiler in here. And anyway it’s right at the beginning and I’ve jiggered the link to skip that, so you should be fine. (If for some reason the auto-skip doesn’t work, start at 0:40.) (Edit: It didn’t.)
Plus, this may be funniest five minutes of dialogue you ever hear*. I was friggin crying by the end.
*Apart from Cave Johnson, but that dialogue is way too spoilery for me to link here.