Think of arrangement as ‘who is playing what’. When arranging a music composition, not only do you select the instrument in which each part is going to be performed on, you can also tweak a bit the melody and the harmony or you can even add new tiny music ideas. So, this is yet another broad music term, as you can do a lot of things while arranging a composition. You could keep the main elements of the harmony but change a few chords. You could keep the melody and add another melodic layer on top, which would give the newly arranged composition a new character. You could simply change the instruments while keeping the melody and the harmony mostly intact, thus creating an entirely different sound. You could add vocals or remove them. You could change the tempo a bit. You could change the genre. You could change how you transition from each music section to the next, repeat certain sections or just skip some entirely. However little the change may seem, it makes a difference. Many composers choose not to arrange their compositions, while others do it themselves. A good arrangement can be a make or break deal, so collaboration is essential between the composer and the arranger.