Dominic Toretto appears at the end of Tokyo Drift to race Sean and to tell him that Han was family, which links the film to the first two films. Spoilers follow.
Here is the ending step by step. Sean and Takashi agree to a downhill drift race down a mountain, and the stake is that the loser leaves Tokyo. Sean uses the skills Han taught him to catch up, Takashi crashes off the cliff, and Kamata honors the bet. Sean stays in Tokyo and is recognized as the new Drift King. Then a stranger arrives in a Plymouth Road Runner and challenges him to a race. That stranger is Dominic Toretto, and his line about Han is the film's hinge, because it ties the Tokyo story back to the series.
The setup matters. Han's death, when another car hits his and it explodes during the chase after his dealings with Kamata, is what gives Dominic Toretto a reason to be standing in that parking lot. In my opinion, that is why the cameo reads as a consequence and not as a bolted on favor. The ending also mirrors the opening. Sean begins the film by losing a street race to Clay in Arizona and wrecking his car. He then loses the borrowed Nissan Silvia race to Takashi and destroys that car too. In the last race the loss is no longer a car but his place in Tokyo, and this time he wins. In my opinion, that reversal is the real payoff, and the cameo is the series' hook placed on top of it.
Production context supports why the mountain race plays as a driving lesson finally paid off. Per the Wikipedia entry for the film, drift expert Keiichi Tsuchiya was brought in to help with the driving because the studio's stunt drivers could not drift. Principal photography ran from August to November 2005 in Los Angeles and Tokyo, and some of the races were filmed in a Los Angeles mall parking lot dressed to look like Tokyo. That means the drifting on screen had a specialist behind it, even where the setting was dressed.
The series order adds a second reading. Tokyo Drift is third in release order, after The Fast and the Furious and 2 Fast 2 Furious, but sixth in chronological order. In story order, the Road Runner scene therefore sits after Fast & Furious 6 and before Furious 7. It could be argued that this placement is why the cameo tells the audience Tokyo Drift was never a standalone side story. If you want the full picture, check the Fast and Furious timeline and read about Dominic Toretto in Fast and Furious. Tokyo Drift has no post credit scene.