You probably dont need to practice modes just yet.
But, for your curiosity, modes are simply the same interval pattern as the major scale but starting and ending from a different note within the scale. For instance, if C Major (Ionian or M7) is CDEFGABC; Then D (Dorian or m7) is DEFGABCD, and so on... Comparing this conversely with the D Major (Ionian) D, E, F#, G, A, B, C#, D; you are flattening the 3rd and 7th notes.
If you notice the 3 part chord (1,3,5) that goes with the D Dorian to be D, F, G, which has a flat third interval compared to the D major scale making it minor.
Again, you probably don't need to practice modes just yet if you are only just starting on arpeggiated 3 part chords, but it might be helpful to remember that the chordal structure of the major scale goes Maj, min, min, Maj, Maj, min, Dim.