#4
It's the root note of the power chord.

For example:
A-----5---------
E-----3---------

That ^ is a G powerchord, because the third fret of an E string is a G note.
#6
Quote by ahhhhxitsxnick
is

D|--5-
A|--5-
E|--3-

also a G then?

Yep. A G5 (G powerchord) has the notes G and D. The only difference between the power chord The_Trooper posted and the one that you posted is that G is in the chord twice.