A Bard (BRD) uses songs to boost the party's stats. Bards have many songs, but only three non-meritable Job Abilities. A bard without an instrument or a player with the support job bard can only keep one song up at a time, as opposed to instrument players that can use two. A party with multiple bards may stack more than two songs on the party as long as they are not the same song.
The Bard job is available after completing the quest Path of the Bard. You must level Bard to 75 for this achievement. For more information on the Bard, look here.