All it means is to play a C note with a G root.
Or in other words, play an ordinary C note but put your 3rd finger on the 6th string 3rd fret, and use your fourth finger to play the 5th string 3rd fret.

It's no big deal... if you wanna keep it simple just play an ordinary C chord.
When you're playing with a band, the bass player will take over playing the G note for you.