This is Biggs Darklighter, Luke’s best buddy growing up on Tatooine. There was a big chunk of story cut from A New Hope where Luke looks up at the sky, sees the Star Destroyer and Princess Leia’s ship shooting at each other in orbit, and jumps in his landspeeder to tell his friends like an excited puppy.
He arrives at Tosche Station (from the infamous line “But I was gonna go to Tosche Station to pick up some power converters!”) and is surprised to find Biggs there, who had just gotten his certification from the Imperial Academy (mentioned in the line “That’s what you said when Biggs and Tank left” when Luke was trying to coax Uncle Owen into applying). Luke drags everyone outside to look but by that time the two ships have stopped shooting, so they write it off as Excitable Dumbass Luke getting his dumb hopes up again and go back inside.
It’s worth noting that Biggs takes the first look through Luke’s binoculars and says it’s probably just a freighter refueling. Having been to the Imperial Academy he’d know damn well what a Star Destroyer looks like and that having one in orbit over Tatooine means serious business is afoot. But he doesn’t mention this and lies, probably in an effort to keep Luke from going “ZOMG ADVENTURE!” and trying to get involved.
When everyone else inside, Biggs and Luke go for a walk and Biggs lets Luke in on a secret: he and a bunch of other Academy grads are going to mutiny and defect to the Rebel Alliance the first chance they get. Luke basically goes “GEE WHIZ!” and Biggs shuts him up. He explains that this is stupidly dangerous and is going to make him a wanted man if he survives, so this is the last time the two are probably ever going to see each other. Luke still doesn’t get it yet and is mostly envious of all the excitement and adventure Biggs is about to embark on.
Fast forward past: Luke discovering real and innocent people get murdered by the Empire (courtesy of Uncle Owen & Aunt Beru’s smoking remains), finding out that dashing rogues can really just be selfish, trigger-happy assholes thanks to Han Solo, and watching the man who opened his eyes to a bigger universe get killed by the monster who Luke thinks murdered his father. His boyish naivety has taken quite the beating. But as he gears up to help attack the Death Star, who should he run into but his best buddy Biggs! How bad can war be when your best friend is at your side?
Biggs gave his life to protect Luke, physically blocking Vader from shooting his best friend for as long as he possibly could. Between that, the reassuring words of Obi-Wan, and the timely return of Han (who Chewie threatened to tear the arms off of if they didn’t go back), Luke learned a critical truth: the universe was a lot darker than he ever realized, but no matter what there is always hope.