When you search by keywords, the results are going to include the words you type, but not necessarily together and in that order. However, when you surround your terms by quotation marks, you are searching as a phrase, so your results will include your words next to each other and in exactly the order you typed them.
Example: "book smart"
If I just type book AND smart, you would get a lot of articles that happen to have both of these words, but if you enclose the words in quotes...now this is a phrase that has a specific meaning.
Phrase searching is also good for searching the Internet for titles or names.
You can also combine phrase searching with boolean operators. Example: "book smart" AND "street smart"