Wikipedia:  What is Hanyu Pinyin?

Tones:

Think about English if it is your mother tongue.  How would you pronounce “man” in these four sentences?

1. “Māān ooh man, is it hot!”
2. “Did you see that, mán?”
3. “Come on, mǎn, you can do it!”
4. “Màn! That’s hot.”

These instances of “man” are spoken with different tones.  In English, tones change the “mood.”  In Chinese, tones change the meaning.

There are four tones in Chinese:  1^st high, 2^nd rising, 3^rd dipping, and 4^th falling. There’s also a neutral tone.