Dominant 7th Chord

This is the 5th chord with an added 7th.  If you are in the key of C major the dominant 7th chord would be G7.  G B D F.

Listen to this song by The Beatles at 1.05 for 2 bars.  The song is in the key of E major and at this point it settles on B7 the dominant 7th then drops to chord IV,  A7, finally going to chord I creating a plagal cadence.