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

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