Chinese languages are tonal. The Chinese language has approximately 400 syllables,whereas English has approximately 12,000, and one of the ways to increase the range of meanings to words is to give syllables different tones. In fact, apparently, it's possible to have as many as fifty words that sound exactly the same, and the only way to differentiate them is with tone and context. The Cantonese dialect has seven tones, and Mandarin has four tones.
In short, the tone in which a word is said affects its meaning. The four Mandarin tones are high, rising, low, and falling.
For example, the word Tung, said in the four tones, has four different meanings:
Tung (high tone) means to succeed
Tung (rising tone) means together
Tung (low tone) means to govern
Tung (falling tone) means painful
You can hear the word ma in each tone at this BBC game, by clicking on the button "learn the tones".
Ma (high tone) means mother
Ma (rising tone) means linen
Ma (low tone) means horse
Ma (falling tone) means to scold