Tones

Unlike any other widely spoken language, Chinese is tonal, meaning that tones accompanying words alter the meaning. English is not a tonal language, but does use intonation. The difference is that while intonation, the way the sound of the voice rises and falls as you speak, can impact the meaning or intention of a sentence, saying a given word differently does not change its meaning. Mandarin Chinese (as opposed to the dialects, which may have different numbers of tones) has four tones, plus a neutral tone. They are fairly clear, that is they are different enough to be able to hear and say the difference.

Tones are important in Chinese in part because of the limited number of syllables. Chinese has about 400 syllables - different pronunciations - and since characters are all just one syllable, that severely limits the possible ways of saying a word. Tones increase the number considerably.
The following short and simple video gives a clear, excellent demonstration.
https://www.youtube.com/watch?v=orx53ieqEI4&