Sound travels in waves and moves in all directions. After leaving a loudspeaker, the waves repeatedly reflect off the six boundaries of a room (the floor, ceiling, and walls) and other surfaces. When one set of waves meet another, a phenomenon called superposition occurs where two or more combine in a way that cancels some frequencies and augments others. There are many ways to refer to this boost and cancellation: peaks and dips, hot spots and dead spots, or if you want to impress your ninth-grade science teacher, nodes and antinodes.
These peaks and dips occur at different frequencies in different parts of the room, and they vary from room to room. They are always present to some degree, even in well-designed spaces and make it challenging to reproduce music and home theater soundtracks accurately. Moreover, peaks and dips are different for every person in the room, because one person may hear a 10 dB peak at 50 Hz while a person sitting three feet (90cm) away may experience a 10 dB dip at the same frequency.