It refers to vocal music performed without any instrumental accompaniment. This could include:
* Solo singing
* Choral singing
* Singing in a group
While it's most commonly used for vocal music, sometimes it can also refer to instrumental music played without any backing tracks.
Here's an interesting fact:
"A cappella" was originally used to describe church music performed by a choir in a chapel, which was often unaccompanied. Over time, the term evolved to describe all types of unaccompanied vocal music.