I'm not really sure why some people think this is LeBron "retiring" or what not... He just finished an all-time great postseason run. He's the clear cut #1 player in the world. I don't think he's giving up anytime soon..
Well, they mean it in the sense first of all that Father Time is undefeated, and that at this age and mileage Bron could drop pretty drastically all at once, despite playing close to his peak right now.
Second reason is the "going to be tough to beat GS anyway for years, so why not raise my family here and prepare for the movie producer second career" one that's been covered.