Good question! In general, an arranger will take a pre-existing piece and rework it for a different style or ensemble. For example, a choir might sing a version of a popular song or a Broadway song, and since it wasn't originally written for a choir to perform, someone has to take the original piece and adapt it for choir. That's where the arranger comes in. So basically, any time a person or group covers someone else's composition and it's not a note-for-note recreation, they're re-arranging it. Hope this helps!