In linguistics, intonation refers to the variation of pitch and stress when pronouncing words. Its function in language is to distinguish meaning. In English, for example, when we are asking questions, we say we use rising intonation. That means that our pitch and stress rise as we get to the end of a sentence. For example, in a question:
What do you think of this answer?
When you say this sentence aloud, listen to how your voice makes a kind of rising pattern - at the end, you emphasize the word "answer", so the pattern is rising.
Now, turn it into a statement:
I think this answer is good.
Say this sentence aloud and you will see a falling pattern of intonation. When you say "good", the pitch and stress go down.
That said, intonation patterns are different in other languages. In French, for example, statements often have rising intonation. This is how a native speaker can easily spot a non-native speaker, because the intonation is not quite right.
Some languages like Asian languages and Native American languages even change the meaning of words by changing the intonation.