The fifth string is tuned to a high G note (the same pitch as G on the first string at the 5th fret). This string is played by the thumb of the right hand and is used as an open string. Here is an exercise to help you get used to playing the fifth string.
It is common to add the high G string to alternating bass patterns. This is demonstrated in the following example. Take it slowly at first and make sure all the notes sound clearly and evenly. Pay particular attention to the right hand fingering.
Once you are comfortable with the pattern on a G chord, try adding some other chords with the left hand, as shown here.