Years before Avengers: Endgame, Iron Man correctly predicted Captain America's final fate. Joe and Anthony Russo's 2019 film culminated the Marvel Cinematic Universe's Infinity Saga, but it also functioned as the swan song to some of the universe's founding heroes. That includes Steve Rogers, who retired from his duties as the Star-Spangled Avenger and passed on the shield to Sam Wilson/Falcon.
As one of the oldest members of the Avengers, Steve had been operating as Captain America since the Second World War. Granted, Steve was frozen in ice for almost seven decades after the events of Captain America: The First Avenger, he immediately went back to duty when he returned. It wasn't an easy process for the super soldier to get acclimated to the modern world; he struggled to get used to the changes around him. This resulted in clashes with other members of the Avengers team, specifically Iron Man.
Over the years, Captain America and Iron Man's relationship went through various stages. Due to their differences in perspective, not to mention a shared history with Howard Stark, they were initially skeptical of each other. They overcame that after working together during the Battle of New York in The Avengers. They differed in process, for the most part, but Steve and Tony were always on the same page with regards to their commitments and responsibilities as superheroes. So while they weren't each other's best pal, they became good friends who genuinely cared about each other. At one point, Tony even correctly predicted Steve's final Endgame fate years before Thanos' arrival: settling down and having a quiet life.
After successfully defeating Ultron in Avengers: Age of Ultron, the original six Earth's Mightiest Heroes went their separate ways. Thor went searching for the Infinity Stones, while Captain America and Iron Man had bid each other goodbye. At that point, Tony's plan was to retire and live quietly with Pepper Potts - even thinking on copying a page from Clint Barton/Hawkeye's book and buying a farm. Steve, on the other hand, was committed to staying active with Natasha Romanoff/Black Widow as they groomed the next batch of Avengers.
As Tony described the kind of life he envisioned for himself, Steve called it the "simple life." Knowing what was lying ahead for the super soldier, Tony offered some words of encouragement to Captain America, saying, "you'll get there one day" - referring to retiring from duty and having the same opportunity to live a normal, family life. At that point, Steve was skeptical of the idea, perhaps because it was no longer possible for him to have that with the only person he wanted to share that life with: Peggy Carter. However, as fate would have it, he was given the opportunity to do just that at the end of Endgame.
This particular conversation was also what Captain America was referring to when he returned to the primary MCU timeline to hand Sam the shield and the superhero mantle in Avengers: Endgame. As he mentioned, he thought it was time to get the kind of life that Tony was talking about, but since Sam wasn't privy to that discussion in Age of Ultron, he didn't fully understand what Steve was saying. All he knew was that he lived a happy and fulfilling life - something he deserves considering his years in the service.