This song will make use of the dominant seventh chord, which almost always resolves upward to a chord one perfect fourth above or downward to a chord one perfect fifth below. This is called a dominant progression, and is used to bring a musical phrase or chord progression to a sound of repose or finality. You will often hear a dominant progression at the very end of a song, especially in classical music.
The numbered brackets over certain measures are related to the repeat sign, and are known as first and second endings. On the first pass, the measure marked with a ‘1’ is played, and then on the repeat, the measure marked with a ‘2’ is played instead, and the song continues from this point.