An arranger is a person who takes a composition and decides what instrument will play what and when. This can be for a recording or for a live performance. Aside from creative reasons, an arranger might arrange the music for the type and/or size of the band or orchestra that will be performing and/or recording it. For example, they may take a piece of pop music and arrange it to be played by a string quartet.
Please note - you can only register an arrangement of a song if it is from the public domain. You may not register any arrangement of a song that is protected by copyright.
You can learn more about registering public domain arrangements here.