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 B major , the dominant 7th chord F#7 resolves to B major

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