Answer by taking a quote from the following article:

A Funny Thing Happened on the Way to the Forum is a musical with music and lyrics by Stephen Sondheim and book by Burt Shevelove and Larry Gelbart. Inspired by the farces of the ancient Roman playwright Plautus (251-183 BC), specifically Pseudolus, Miles Gloriosus, and Mostellaria, the musical tells the bawdy story of a slave named Pseudolus and his attempts to win his freedom by helping his young master woo the girl next door. The plot displays many classic elements of farce, including puns, the slamming of doors, cases of mistaken identity (frequently involving characters disguising themselves as one another), and satirical comments on social class. The title derives from a line often used by vaudeville comedians to begin a story: "A funny thing happened on the way to the theater".

A Funny Thing Happened on the Way to the Forum opened on Broadway on May 8, 1962, at the Alvin Theatre, and then transferred to the Mark Hellinger Theatre and the Majestic Theatre, where the show closed on August 29, 1964, after 964 performances and 8 previews. The show's creators originally wanted Phil Silvers in the lead role of Pseudolus, but he turned them down, allegedly because he would have to perform onstage without his glasses, and his vision was so poor that he feared tripping into the orchestra pit. He is also quoted as turning down the role for being "Sgt. Bilko in a toga". (Silvers eventually played the role -- wearing his glasses -- in a 1972 revival. In the film, he played Marcus Lycus.) Milton Berle also passed on the role. Eventually, Zero Mostel was cast. During the out of town pre-Broadway tryouts the show was attracting little business and not playing well. Jerome Robbins was called in to give advice and make changes. The biggest change Robbins made was a new opening number to replace "Love Is in the Air" and introduce the show as a bawdy, wild comedy. Stephen Sondheim wrote the song "Comedy Tonight" for this new opening. From that point on, the show was a success. It was directed by George Abbott and produced by Hal Prince, with choreography by Jack Cole and uncredited staging and choreography by Robbins. The scenic and costume design was by Tony Walton. This wardrobe is on display at the Costume World Broadway Collection in Pompano Beach, Florida. The lighting design was by Jean Rosenthal. Along with Mostel, the musical featured a cast of seasoned performers, including Jack Gilford (Mostel's friend and fellow blacklist member), David Burns, John Carradine, Ruth Kobart, and Raymond Walburn. The young lovers were played by Brian Davies and Preshy Marker. Karen Black, originally cast as the ingenue, was replaced out of town. The show won several Tony Awards: Best Musical, Best Actor, Best Supporting Actor (Burns), Best Book, and Best Director. The score, however, was coolly received; it was Sondheim's first musical on Broadway in which he wrote both the music and lyrics, and did not earn a nomination for Best Original Score.

What other achievements were there?
The biggest change Robbins made was a new opening number to replace "Love Is in the Air" and introduce the show as a bawdy, wild comedy.