D Major
Major reference↓ same tonic, selected mode
D Ionian
Parallel mode↓ same pitch collection, reordered around its parent Major tonic
Compare a mode directly with the Major scale on the same tonic, then see the parent Major scale that contains the same notes.
Try this: Pick any tonic, then move through the modes in this order:
Lydian → Ionian → Mixolydian → Dorian → Aeolian → Phrygian → Locrian
Pay attention to what changes each time.
If you have an instrument handy, play each scale as you go. Does the sound seem to get progressively darker?
Highlighted notes show what changes between the Major reference and the selected parallel mode.