Nothing like a near-death experience to make you see things differently. After Clarence finishes up his life review and George decides to live again, George runs home to find the bank examiner and sheriff waiting to arrest him for mishandling his customers' funds. Even though he's not worried anymore after his epically perspective-altering experience, all of the townspeople chip in their dollars to bail him out.
When this happens, George realizes what's really important—the love of his family and community. Suddenly, his modest little life in ordinary old Bedford Falls is more precious than anything. This last scene sells the essential message of the film.
A parallel plotline resolved at the end is that Clarence, the relative newbie angel who's modest and a little insecure about himself, gets those wings he's been hoping for. It goes to show that even in heaven, you don't have to do flashy stuff like create the universe in seven days flat to get your reward. Helping one guy to appreciate life is all it takes. George smiles when his daughter hears that bell ring.
Didn't we promise that nice guys never finish last in a Capra film?