Stress and Intonation are important to the meaning of spoken communication. For example, listen to a speaker ask a question for the first time.

You can hear the pitch of his voice starts relatively high and falls at the end, finishing relatively low. This intonation pattern is shown here using an arrow.

In another example, if the same speaker should ask the question for a second time for some reason, then the voice falls on the word where and rises again towards the end of the question. The speaker communicates with the same words to the listener, but the intonation and stress gives a signal that they should know the answer.

Stress can be important in the communication of meaning. In the following sentences, the most stressed syllables are in capitals. You can see that changes to the stressed syllable in the same sentence changes the meaning of the communication in various subtle ways.
The implied meaning is given in brackets after each utterance.

I’d like a cup of herbal TEA. (A simple request.)
I’d like a cup of HERbal tea. (Not any other sort of tea.)
I’d like a CUP of herbal tea. (Not a mug.)

The first example is like the default choice; a first-time request, while in the other two examples there is an apparent attempt to clear up some misunderstanding between the speaker and the listener. You can notice how the speaker’s voice falls on the syllables which are’ in capitals, demonstrating how intonation and stress are strongly linked and can influence the meaning of the phrase.