The verses of the song have a chord progression of G, Em, C, G/B, Am.

Not sure how to play a G/B?

Imagine playing the G chord without using the low E string. In effect, the B on the 2nd fret of the A string acts as our lowest note. This chord is used in many songs as a transition chord between the C and Am chords (and vice versa). For example, C, G/B, Am, or Am, G/B, C.