This feat exists simply to add the supplementary information provided in Chapter 2 of Xanathar's Guide to Everything to a character's sheet so that they can more easily flavor their interactions with their tool proficiencies. It does not function as a standard feat, and is not to be taken as part of an Ability Score Increase.

Proficiency with a musical instrument indicates you are familiar with the techniques used to play it. You also have knowledge of some songs commonly performed with that instrument.

History. Your expertise aids you in recalling lore related to your instrument.

Performance. Your ability to put on a good show is improved when you incorporate an instrument into your act.

Compose a Tune. As part of a long rest, you can compose a new tune and lyrics for your instrument. You might use this ability to impress a noble or spread scandalous rumors with a catchy tune.