In a home system, you hear sound direct from speakers. You also hear sounds emitted by the speakers to hard reflective surfaces then reflected back to you. Walls, floor, ceiling, etc.
Such reflective areas behave like mirrors. If sound where visual, your ears will see a reflection of the speaker on these reflective surfaces.
Two things can happen to the sound. Reflections can reach your ear in-phase or our of phase. In-phase, boosts sound. Out-of phase reduces sound.
In a badly reflective room, you can get peaks and valleys in your sound response curve.
That's putting it simply. Other room effects are standing waves, echos, peakiness (boominess, shouting etc.), and resonance (vibrations.)
