English speakers may use intonation when speaking in a way that can add colour and in some cases convey extra levels of meaning to an utterance. E.g. if you said "The car is red" with a rise in pitch at the end of the sentence then that might imply a question; having the pitch fall at the end would be more likely to imply a statement. However, this convention isn't universal (e.g. AQI).
Most languages in the world are "tone" languages, i.e. the pitch of the voice when expressing utterances can change the meaning of the utterance. In some cases the meaning of individual "words" changes depending on their tonation, and in some cases aspects such as tense of a sentence can be modified.
There are two ways in which tone is used to modify meaning:
Most speech utterances exhibit downdrift, where the pitch gradually lowers as the speaker runs out of breath. E.g. in a tonal language, the "high" pitch being used by the time the end of the utterance is reached is likely to be a lower frequency than it was at the start of the utterance.
Although a particular word may be defined as being uttered with a given tonation, tonal sandhi may change this. This is "the fusion of sounds across word boundaries, or alteration of sounds due to neighboring sounds, or because of the grammatical function of adjacent words...Most tonal languages have tone sandhi, in which the tones of words words are altered in complicated ways. For example: Mandarin has four tones, a high monotone, a rising tone, a falling-rising tone, and a falling tone. In the common greeting 2ni 3hao, both words would normally have the falling rising tone. However, this is difficult to say, so the tone on 3ni mutates into 2ni." (source: Wikipedia).
Useful book readings for this lecture: