A E Major chord contains the notes E, B, G#. It is built from the major formula 1 - 3 - 5, with E as the root.
How does E major relate to barre chords?
The E major shape is the template for every E-string root barre chord. Move the same finger pattern up the neck and add an index-finger barre behind it, and you can play any major chord — which is why E major is worth learning cleanly early on.