For a start, let's talk the major scale modes (there's modes deriving from other scales, too, but the most common ones derive from the major scale).
Now, what you may still find in many articles, lessons, videos and what not on that subject, is things like that: "D dorian is the C major scale played from D to D". There couldn't be greater nonsense. In case you have a nice C major chord and play a C major scale starting from D and going up an octave to the next D, you're still in C major! So, once you read that kinda thing as an explanation, continue elsewhere, delete the bookmark, write hate comments, etc.
Ok, looking at a mode like that does have its purposes, but they're strictly analytical, nothing else. Simply because you analyze each mode from its root.
Whatever, the definition should rather be: When you play a C major scale in a "D context" (ideally a D minor context, but a drone D note would do), your entering dorian territory!
The same goes for all the other modes. They're superimposing a major scale over a scale degree root which is supposed to be established as a kind of tonic chord on its own.
In case anyone's interested how to actually internalize this kinda stuff (which isn't really tough), I'd happily continue, need to prepare some food for the kids now.