The arranging credit can cover a huge amount of territory, from adapting a piece of music for a particular instrument or ensemble to creating a version of a piece or song with an entirely new feel.
Since you specified seeing the word on a score: one example might be taking a popular song and arranging it for a marching band. The arranger will take the rhythms, melodies and harmonies from the original track and distribute them to the instruments of the band so that it's recognizably the same song, although obviously with a very different sound.
Another example: I was once given the assignment of turning a musical theatre song originally written as a disco number into a piece that sounded like 1930s swing.
Both of these would be considered arranging.