There are 4 tones in the standard Chinese pronunciation, which are represented in pinyin by marks above the word, and are read from left to right. The tone of a word describes how the pitch of the voice changes as the word is spoken. The four tones are very important in learning Mandarin Chinese, since the same syllables with different tones may lead to different meanings. It may cause misunderstanding if you pronounce the four tones incorrectly.
The following McDonald symbol illustrates the four tones of Chinese Pinyin：
Tone 1 – Flat
The mark in straight line indicating the word is pronounced with a flat and unchanging high tone.
Tone 2 – Uphill
The mark going uphill indicating the word is pronounced in a rising tone.
Tone 3 – Valley
The mark going down and then up indicating the word is pronounced with a falling and then rising tone.
Tone 4 – Downhill
The mark going downhill indicating the word is pronounced with a falling tone.
p.s. When a word has no tone or mark above the word is pronounced in an abbreviated manner with no emphasis. It is normally used at the end of the question.