The modes are simple once you learn their whole and half step patterns.
Using the major mode Ionian as your starting point all other modes are just different starting points of the major scale.
The modes go - Ionian(major), Dorian, Phrygian, Lydian, Mixolydian, Aeolian(minor), and Locrian. 1 2 3 4 5 6 7,(8 is 1, a.k.a. an octave)
So based off of a Major scale(Ionian), 3 to 3, or 3 4 5 6 7 1(8) 2 3, is Phrygian. 2 to 2 would be Dorian, ect ect.
Good break down.
The summary at
4.2 helps know what modes to use where (a little).
ii-V-I refers to the diatonic chord progression built off a particular key. The little ii shows that is a minor 2 chord, into a major 5 chord to a major 1 chord in what ever key you are in.
Diatonic major chord progression is -
1(Major) – 2(minor) – 3(minor) – 4(Major) – 5(Major) – 6(minor) – 7(diminished) OR
I – ii – iii – IV – V – vi – vii°
Diatonic minor chord progression is -
1(minor) - 2(diminished) - 3(Major) - 4(minor) - 5(minor) - 6(Major) - 7(Major) OR
i – ii° – III – iv – v – VI – VII
Just like the minor(Aeolian) is the 6th tone in the major(Ionian) scale, the same is true with the progression above. If you knew the major progression, then just counted 6 to 6, you will have the minor progression.
All this is based off of Triads. Triads give us chords and LOTS of other stuff.
Best thing you can do as a guitarist who wants to understand the ins and outs of music theory is not to just memorize the fingerings and patterns on the fretboard,
learn the whole and half step patterns of the modes and their variations, like harmonic and melodic minor. That should give you quite a lot to work on. Just do one thing at a time.
Btw, my explanation probably is vague at best, so get to googling and good luck.