What Does it Mean to be an "Arranger"?

They pull the strings in a performance (no pun intended).

An arranger is a person who decides what instruments will play what parts — and when — in a composition, either for a proper recording or live performance. 

Aside from creative reasons, an arranger might rework a song for the orchestra or band that is performing it. A pop song played by a string quartet would be one example. 

Since they’re often a key part of the creative process, arrangers will sometimes receive a songwriting credit when they’re involved in creating a new composition. It depends on the deal they struck beforehand. 

Regardless, you can only register the arrangement of a public domain song. You cannot register the arrangement of any song that is protected by copyright

Learn more about registering public domain arrangements here.