in short every scale has 7 basic modes... three are major three are minor and one is diminished the 1st 4th and 5th mode are major, the 2nd 3rd and 6th are minor and the 7th is a diminished.
the modes use the same note as the scale you are playing but start on a different note and give a different feel to what you are playing. easy example in the key of c.... c d e f g a b (the c ionian mode) is just the c major scale.
d e f g a b c (the d dorian mode ) uses the same notes of the scale
but you anchor your riffs around d instead of c. very different feel.
it is called the d dorian as it is the 2nd mode of the c major scale.
just a short example... but the links the above poster gave are good explanations.
