This is from the Theory section on Chords.

Let's play the most common E major chord. This is what we call the 'open' E major chord because it is played at the lowest position on the neck, allowing us to use as many open strings as possible.

All basic major and minor chords consist of 3 notes (the 1st, 3rd and 5th notes), taken from their parent scale. So we get these 3 notes from the E major scale to give us the E major chord:

E Major Scale: E F# G# A B C# D# E

E Major Chord: E - G# - B

Looking at the chord in this ActionTab we can see the notes are played in the following order (from Low E string to high E string):

Low E : B : E : G# : B : E

Placing our fingers on these notes and strumming across the strings means we play E, G# and B together to get the chord.

Of course, some notes we are playing more than once and this adds extra depth to the chord.

Generally with open chords we prefer to have the 1st note of the chord (the root note, in this case E) in the bass. In other words, we prefer to start the strum with the Low E at the beginning, rather than starting from B or G#. Although with some chords this isn't easily done due to fret positions.

Also, just plain musical preference will mean that musicians will alter their chord notes too. However, the chord 'shape' you see in this ActionTab is the most common E major chord. By 'shape' we mean how the fingers are organised / positioned on the fretboard.