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 D major , the dominant 7th chord A7 resolves to D major

It's also possible to use the 7th chord as the dominant of other chords. For example, in G major, we could use A7 as the "dominant of the dominant". The notes of the A7 lead to D, which itself can then lead as a Dominant chord back to the tonic G.