When PG is referring to 1 3 5, he is referring to the 1st 3rd and 5th notes of the Major Scale.
Think of the Major Scale as the DNA that builds the Major Chord
If you have the C Major Scale:
Then a C Major Chord would be:
CEG (135) of the Scale.
Notice how the Chord is built: pick a note, miss a note etc (pick C, miss D, pick E) if you do this from every note in the Scale, you get all the Chords in the C Major 'Key' or 'Chord Family' - made up of the DNA which is the Major Scale
This is how you fund chords that work together for songs too!
Hope this helps!