Examples of use
The 7th chord, also known as the Dominant 7th is so-called because it is formed from the 5th degree (or Dominant) of the scale. So in Ab major , the dominant 7th chord Eb7 resolves to Ab major

It's also possible to use the 7th chord as the dominant of other chords. For example, in Db major, we could use Eb7 as the "dominant of the dominant". The notes of the Eb7 lead to Ab, which itself can then lead as a Dominant chord back to the tonic Db.