When they talk about the 'root of the chord' they mean the note for which the chord is named. A G note is the root of a G chord, etc. The 5th of a G chord would be D. This is because a G major triad is made up of the notes - G, B, D.

G - 1st (root)
B - 3rd
D - 5th

B and D are called the 3rd and the 5th because of their places in the Gmajor scale. Major triads are always the 1st, 3rd, and 5th tones of the major scale, but none of this matters unless you know the notes of the fretboard.

I suggest you get one of those charts that shows the names of each note on the fretboard. Along with a little theoy, you'll be able to figure out what chords you're playing anywhere on the neck.