Hexspa's explanation of modes above is, from my understanding, correct but the subject is somewhat abstract so depending on what stage of the game you're at, you may consider other points of reference. I'm sure Ethan has explanations of room modes if you search his pages (sorry, I don't have a link handy at the moment:(. But...the first reference I learned about room modes was from this book (an earlier edition for sure) www.amazon.com/Master-Handbook-Acoustics-Alton-Everest/dp/0071603328 Look for the Chapter on Modes or Room Modes.
Let me know if I can explain it better for you. Maybe you have something that seems left out or unclear. One day I plan to make a video explaining this but I don't feel like I have a strong enough concept yet.