MajorMajora

I understand music theory. Decently, at least. I was a trumpeter in my school band for 7 years, and I started composing music and I like to think I have a good enough grip on it. But the problem is I don't know the lingo. I'm able to find good guides on things I basically understand already, but I have a question on modes. Not the modern ones, like Ionian, Locrian, etc.. I understand those. My problem is with these old greek scales and church modes and what not. I look things up and I can't figure out what anyone is talking about. Not because it's complicated stuff, but because thye use words like "diatonic" and "perfect fourth" and "4:3 perfect resonance" or something like that, which I never had to learn. I'm sure they just refer to concepts I'm perfectly familiar with, but I don't know lingo. Could someone explain these things to me in plain english?