And this one is both far-reaching and just for fun. The Watchers are an immortal race of beings who watch over all of time and space - every parallel universe, every populated world, and appear particularly to observe and chronicle any major events. When a Watcher, usually one known as Uatu, showed up in the pages of Marvel Comics, you knew something big was going down.
The implication here (and in the earlier scene), then, is that Stan Lee has been a Watcher all along, in human form, thus making all his appearances in all Marvel movies, from Marvel Studios, Fox, and Sony Pictures, one long canon appearance. That's something fans have speculated and "head-canoned" for some time, and here Gunn plays into that wonderfully. It's just all too much fun. If they're not saying he's a Watcher himself, then he's some kind of extra-dimensional being who can tell them about all of his lives. Still cool.
Now, here's where this also gets into major significance territory: Uatu the Watcher and the race in general are almost certainly ear-marked as Fantastic Four characters, meaning their film rights should belong to 20th Century Fox under that license, which is rumored to cover thousands of characters. Nearly all their major appearances have been in Fantastic Four-centric books and stories and Uatu in particular played a major role in the first coming of Galactus to Earth.
Does this mean Fox and Marvel played nice in order to give Stan his due? Is it a sign of something bigger to come with the Fantastic Four and Marvel Studios that every fan ever has been begging for? Or was this race just randomly left out of an insanely detailed contract that even prevented the first Guardians movie from calling their cannon-fodder aliens "Badoon," despite that race's link to the franchise, simply because they first appeared in a Fantastic Four-related comic (Silver Surfer, for you trivia buffs)?
That last option is highly unlikely, but this is all still (informed) speculation.