First, what it means: on the review-aggregator site, a good review is denoted by a Fresh red tomato. A bad review is denoted by a Rotten green tomato. To receive an overall rating of Fresh, a movie's "Tomatometer" score must be at least 60%. Otherwise, it is Rotten. Movies that have a Tomatometer of 75% or greater and at least 40 reviews from Tomatometer critics, including at least 5 from Top Critics, receive a special designation of Certified Fresh.
It's weighing in at 77%, to be precise, along with an 84% favorable rating from audiences so far, which isn't bad...but it's nothing like recent comparably-huge comic book movies.
Marvel's The Avengers opened with 96% on Rotten Tomatoes and remains the highest-rated of the films at 93% now. The Dark Knight Rises began its opening weekend at 87%, and has stayed there. Both of those films had early audience scores in the 90s.
Iron Man 3's reviews are more comparable to Dredd 3D and The Amazing Spider-Man, both of which are in the '70s on Rotten Tomatoes. Dredd carries a 78% rating while The Amazing Spider-Man has 73%. Both of them have shifted a little since their initial release, with The Amazing Spider-Man taking on an additional point from its early 72% rating (and carrying about 81% in the audience scores) and Dredd dropping substantially from pre-screenings, as the first batch of ecstatic reviews had the film debuting in the mid-'90s. By opening weekend it settled in at 77% (tied with Iron Man 3) once the mainstream American critics got hold of it.
It's interesting that this film didn't connect with critics. The whole undertaking was a bit of a risky move by Marvel, tinkering with the characters' backstory, the familiar Marvel movie structure and crafting a movie that was had more of an "auteur" stamp on it than anything that came before. Kiss Kiss Bang Bang and The Brothers Bloom have almost as much in common tonally with Iron Man 3 as do the previous Iron Man movies, and those movies were critically beloved. This writer would be lying if I didn't say that my first time watching the film, I expected many reviews to be ecstatic.
The 77% rating, if it holds, will mean that Iron Man 3 is the second-best-reviewed of the series, following after Iron Man at 93% and coming in ahead of the 73% for Iron Man 2. Whether the score will sink, rise or remain steady remains to be seen; the comics movies that start in the 70-percent range seem to stay there, often with a slight uptick over time, but obviously huge blockbusters like Iron Man 3 tend to drop as early screenings--even those for the media--are full of adherents.