Brent Butterworth, a writer over at Sound & Vision, has written a very easy to understand explanation of what the 'Schroeder' frequency is. The Schroeder or 'transition' frequency is a critical concept to understand because it explains a lot about how sound behaves in rooms, how we measure / analyze them and how we treat them.
I'll just make his perfect prose more complex than it needs to be, so I'm just going to post his first paragraph for you all to read!
You have at least two listening rooms. Even if you live in a studio apartment, you have at least two listening rooms. Well, in a sense. Every listening room is, in essence, two listening rooms when you look at it from the perspective of sound.
To midrange and treble frequencies, your listening room is like a billiards table. Like billiard balls, mid- and high-frequency sounds tend to bounce all around your room, until they run out of energy. Because of this frenetic reflection, midrange and treble frequencies spread pretty evenly — or diffuse — throughout a room.
To bass frequencies, your listening room is like a beer bottle when you blow across its top. In other words, it’s a resonator. Sounds whose wavelengths match the dimensions of the room will resonate — in other words, they’ll be amplified. Sounds whose wavelengths don’t match the dimensions aren’t amplified. Depending on where your speaker is placed in the room, and where you are placed in the room, some of the bass sound waves will reinforce each other, while others cancel each other out. Move to a different spot in the room and different frequencies may be reinforced or canceled.