Well take an F major scale. F, G, A, Bb, C, D, E.
Bb is the forth note of the scale. So F to Bb is a forth. F to G would be a second, because G is the second note of the scale.
So if you knew that Lydian is the 4th mode of the major scale, you would know that Bb Lydian is the same as F major because Bb is the 4th of F in F major. If you knew that Dorian was the second mode of the major scale, you would know that G Dorian is the same as F major, because G is the second of F major.
If you're playing in Bb Lydian and want to know what major scale is its parent, if you knew that Lydian was the 4th mode, then you know whatever note the mode starts on is also the 4th of the parent major scale, so you could simply count down.
Thanks for all of your help, I get it now.