Emphasis
Another important way that English speakers use intonation is to mark emphasis. Here, it gets tricky, because there are two things going on - intonation and volume:
Intonation: how high or low your voice is
Volume: how loudly or softly you're speaking
Emphasis involves a change in both intonation and volume. Typically, English speakers will mark an important or emphasized word in a sentence not just by making it louder, but also by raising and then quickly lowering pitch. For example, consider the following sentences:
'I want chocolate ice cream for my birthday.' This sentence stresses 'chocolate,' so the speaker is focusing on the proper flavor: chocolate, not vanilla or strawberry.
'I want chocolate ice cream for my birthday.' This sentence stresses 'ice cream,' so the speaker is focusing on the type of food: chocolate ice cream, not chocolate cake or chocolate pudding.
'I want chocolate ice cream for my birthday.' This sentence stresses 'birthday,' so the speaker is focusing on the occasion: a birthday, as opposed to Christmas, Easter, or another type of holiday.
These types of intonation cues can help you focus on the most important words in a sentence and get a better idea of what the sentence is actually about. For example, a sentence where 'chocolate' is stressed might indicate that two speakers are having a conversation about what kind of ice cream they want, while a sentence with 'birthday' stressed might indicate that the conversation is about what foods are best on what holidays.