2-5-1 is a chord progression. It's based on the 2nd chord - the 5th chord and the 1 chord of any particular key.
In G Major for example - the 2-5-1 chords would be:
2= A minor (cuz A is the second note in the G Major scale - and if you make a chord from that note in the key of GMajor it will be a minor chord)
5 = D7 (cuz the 5th note of Gmajor is D and when you make a chord out of it it has a mjor 3rd and a minor 7th so it is called Dominant 7)
Wanna guess what the 1 chord in GMajor will be?
Yup - G Major. Now - if you change the key to C major - your 2-5-1 chords then become D minor - G7 - CMajor.
Chords can be made off all 7 notes in the major scale. Each of these chords also implies a mode or seperate scale that can be played over it for melody and bass lines.
So in the 2-5-1 example above - your modes are A dorian (minor) - D Mixolydian (dom) - and G Ionian (major)
The other modes are very useful for creating lines over extended and altered chords. Since chords are created by taking every other note in the scale and playing them at the same time - you can create simple chords - 1, minor 3rd and 5th= basic minor and you would use the dorian mode over this. Or 1 Major 3rd and 5th would be basic major and you would use the Ionian mode over that chord. But throw in a sharp 4 on your major and now your lines will sound better if you play in the lydian mode.
My suggestion would be to find an instructor who can give you excersises that will help you learn the major scale across the entire neck in all 12 keys - and which modes of the major scale are impled in various chord symbols.
It sounds like a lot of ground to cover - but once you get some of this under your fingers you'll begin seeing huge jumps in your abilities.
I know you're still wondering how to use modes to solo. All I can say is that you wont really appreciate the answer until you internalize the major scale forms across the fingerboard in all 12 keys and learn the triads (the 1-3rd and- 5th notes of every mode.
Then you will SEE and HEAR how the modes work. They are embedded in the major scale and cannot be seperated from it. To do so is to only SEE and HEAR a fraction of the available story. AND you will know the chord types over which to use the various modes. I'm sure others have loads of info for oyu as well - and there's tons of great posts about this topic in these forums - spend some time searching and reading.
