The Salem Witches' Institute is a magical institute, possibly a school, presumably located in Salem, Massachusetts. Based on its name, it presumably only accepts female members. Some of the institute's members attended the 1994Quidditch World Cup, and were gossiping happily in between two tents bearing their school's name.
Behind the scenes
Some fans believe this to be the American wizarding school that J. K. Rowling claimed in an interview would be mentioned in Harry Potter and the Goblet of Fire. However, assuming she meant "American" to mean the Americas in general rather than merely the United States, another American school is mentioned in the same book. Additionally, the only known members of this institution are described as middle-aged (though they could be teachers or alumni).
Assuming it is not a school, the institution may be a magical counterpart to the Women's Institutes, though these only exist in Great Britain in the real world, though muggles would not actually know if there was a magical women's institute in America because it would have all kinds of muggle repellents on it.
The banner which displays the school's name is described in Harry Potter and the Goblet of Fire as a "spangled banner", possibly the American Flag, which is "spangled" with stars and referred to in the American National Anthem as "the star-spangled banner".
If it is indeed a school, the name "witches institute" would suggest it is a girls only school, probably indicating that there is indeed a boys school too.
The irony here is that the school, based in Salem, Massachusetts, was also the site of the infamous Salem Witch Trials.