Musical groups commonly record a group of songs, which are then offered to the public in what is known as an album, or, these days, simply as a CD. For example, one might write: Cowboy Junkies is set to release a new album (or a new CD) which is their first new release in several years.
A single refers to a single passage of music, usually a single song, which was one of the passages or songs recorded when the musician or musical group recorded the group of songs which will be offered to the public as a new CD. So, to release a single means to offer the public one of the newly recorded songs by itself, separately from the new CD or album.
One popular method by which bands can secure publicity for a new album is to release a single on their webpage.
This means that the musical group, the band, will promote sales of the new album or the new CD by issuing a single song in order to spark the interest of the buying public.