A pre-release single is a full, finished track pulled from an album that has not come out yet. It goes up on streaming platforms ahead of the album's own date, and it usually arrives with its own music video or a visualizer. Because the whole song is there from the first day, you can play it on repeat right away, and that is the main thing separating it from a teaser. A teaser is a short clip, while a pre-release single is the complete song.
A pre-release single is also different from a standalone single, even though both are full songs arriving outside an album's title-track moment. A standalone single stands on its own, with no album behind it, so there is nothing bigger that it belongs to. A pre-release single works the other way around: it is announced together with the album, and the official post names that larger release directly. If a song turns up with only a title and a date and no album mentioned anywhere, it is more likely a standalone release.
A pre-release single is usually not the title track. Labels normally hold the title track back for the album's release day, since that is the song built to carry the main promotion once the full record lands. It helps to read the pre-release single as an early, extra offering rather than the lead itself. Labels use the format because it gives the album a first listen before release day and puts something playable on streaming during the weeks of teasers.
You can usually tell which kind of release you are looking at from the wording of the official video description. For a pre-release single, the description says so directly, names the album it belongs to, and gives that album's date and time. When only a title and a date appear with no album named, the song is more likely a standalone single. One practical note: a pre-release single is normally included on the album once the album arrives, so saving it early does not mean you are getting a song that will not show up again.