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 A major , the dominant 7th chord E7 resolves to A major

It's also possible to use the 7th chord as the dominant of other chords. For example, in D major, we could use E7 as the "dominant of the dominant". The notes of the E7 lead to A, which itself can then lead as a Dominant chord back to the tonic D.