A&R literally stands for "Artist & Repertoire".
In a young industry, this could be regarded as an old-fashioned expression as it harks back to the days when the A&R person - who was often a Record Producer - would find an Artist, sign them to their Record Company and choose the Songs ("repertoire") they will record.
These days, A&R departments are less involved in every aspect of an artists career with their Producer and sometime Manager being more involved in the choice of songs.