A “mode” of the major scale is a variation of a set of notes, starting on a different root than the major scale to obtain a different harmonic substance.
Modes are commonly used in all styles of music, predominantly Jazz and Classical but there are plenty of cases where they occur in all genres.
Modes are commonly misunderstood but very easy to realise by looking at this chart below:
There are seven modes as you can see from the above. The order of the modes never change so your 5th mode of the major scale is always going to be Mixolydian for example. The only thing that changes are the keys and the notes that correspond with that particular key. To discover the sounds of each mode, you can easily go to a piano and play every white key from C to C - that would be the first, "Ionian". Then play every white note from D to D - that would be your "Dorian", then E to E - "Phrygian" and so on.
Transfer this knowledge to any instrument and learn how each mode sounds, not how they look. Once you hear that specific sound, the modes become very valuable in improvising, composing etc.
Now you want to know how to recognise them if you have a chart or sheet music put in front of you.
Let's say that your key is written as C because there are no obvious sharps or flats, but your progression starts with a Dm7 chord, and is primarily fitted around that e.g. (Dm7 | Bhalfdim | Fmaj7 | Dm7). Does this mean that your key, or better put, "mode" is Major/Ionian?
Not straight away. If we quickly analyse this we can see that D Dorian would be a wise choice to write a melody on/improvise over, simply because your piece is based around the root of D. Dm7 is the second chord in the key of C and D Dorian is the second mode.
Similarly if your key signature states 2 sharps (D major) but everything resolves around a G major progression, then G Lydian is the best choice because it is the fourth mode and is four steps up from D in the key signature.
If you would like any more help on modes or music theory in general, please contact me to discuss. Remember that I am available for lessons in Caerphilly, Pontypridd, Tonypandy (RCT), Aberdare, Cardiff and surrounding areas. I am also available for Skype and Facetime lessons.