Sentence stress is the pattern of stressed and unstressed words across a sentence. Normally this emphasis is on words that carry important information, although this can change significantly, depending on the specific meaning the speaker wants to communicate.
'She bought a new car' probably has main stress on 'car' and secondary stress on 'bought'.
In the classroom
The way stress moves in order to change the message is an extremely important part of pronunciation, and many teachers spend a lot of time working on this aspect. For example, in the sentence above main stress could move onto 'she', 'bought' or 'new' and change the meaning considerably.