These new chords are really called dominant seventh chords. They are a type of Major chord but they have more notes.

A basic major or minor chord contains three notes. (we play four on the ukulele but two of the notes are actually the same, try playing an F Major and see which two strings play the same note.)

7 Chords add an extra fourth note, this note makes the chord sound a little unsettling.

Because the chord sounds uncomfortable it needs to resolve into either a Major or Minor chord to feel complete.