An open string is a string which is played without using your left hand fingers. When you play the open third string you are playing a G note.
Hold the pick in your right hand. Pluck the third string over the sound hole in a downward motion of your pick. You are now playing the note G. Play four G notes.
Remember to play downwards.
The note G is shown on the guitar fingering diagram. Because it is played on an open string, the letter G is written above the nut in a circle.
Use the tip of the pick to play the string.