The Avengers opened in May, and went on to become the third highest grossing movie of all time, both in domestic and international box office receipts. However, now it is just $40 million away from breaking Titanic's record and becoming the 2nd highest grossing domestic film of all time. There's little chance of the movie managing to break that 40 million with its current theater count of just 142 screens - thus, the studio behind 'The Avengers' has realized that since this weekend has an extremely weak frame of films and it's Labor Day, that they would re-release the movie and capitalize on still-existing fanboy ferver.
The movie will be released for one week only, from August 31 to September 6th. There is a high possibility that the film will make it to that coveted number #2 spot, and knock James Cameron off his silver crown (although the gold will probably remain his forever, since Avatar's grosses likely won't be beat in a very, very long time.
Studios generally consider domestic box office as a stronger indicator of a film's quality and performance, as more often than not, international performance is dependent on how many markets the film is released in, whether it's in 3D or not, what stars are in the movie, and general factors that may not be equal for all movies.
In this case, all three movies currently at the top for the international box office are the same movies that are at the top for the domestic box office (details at Box Office Mojo). However, if Avengers were to move to second place in the domestic box office list, it would show that the Avengers was a truly beloved film among American filmgoers, more so than Titanic. Of course, this isn't definitive as a lot of factors have to be taken into consideration - Titanic was released more than a decade ago, and was also not released in 3D.