Chris Pratt was hanging out on the set of the Avengers when he decided to film a few moments of everyone standing around. The set had a very strict no cellphone policy but that wasn't going to stop him.
Warning: SPOILERS AHEAD

It's just a very casual instance of everyone hanging out waiting for the next scene to be shot. The spoilers being some of the cast members vanished at the end of Avengers: Infinite War. But this also could have been a flashback scene? I'll never tell.

In the scene you see the characters in make-up but also wearing jackets. Chris Evans (Captain America) gives off a goofy smile and even comes in for a 'd*** shot'. Sean Gunn lets him know he's probably the most famous person there. And even Jeremy Renner, when the cameras aren't rolling, is acting very 'Hawkeye' like.

All in all, a very fun video.