History
From its founding in 1891 through 1924, the French National Championship was exactly that - purely national, open only to members of French tennis clubs.
Largely because of the Davis Cup, though, tennis became a truly international sport in the early 1920s, and the French tournament was opened to other nations in 1925. Attendance increased immediately, and in 1928 a new stadium was built at Porte d'Auteuil, just outside Paris, to host the event. It was named for a famous French flier, Roland Garros.
The tournament became the French Open in 1968, when professional players were admitted for the first time.
The French Open is the only major tournament played on clay courts. As a result, it isn't generally considered quite as important as the other three majors.