Why use it?
Some search tools are less intuitive than others, and this is especially true when searching for phrases. A phrase is a combination of words in a specific order with a specific meaning. For example, swine flu is a phrase -- these two words must be together and must be in this order to have meaning.
When searching for a phrase, such as swine flu, some search tools will require the user to put quotation marks("") around them to keep the phrase intact.
How it works:
Using the example above, if you were doing a search on swine flu, you would use the quotation marks in the following manner:
"swine flu" AND outbreaks
Not every search tool requires the use of quotation marks as some will automatically assume two words together not separated by AND, OR, or NOT mean you are searching for those terms as a phrase.
Refer to the help guide of the search tool you are using to determine whether or not it is required.
Think carefully before using quotations, as there are some cases where your search terms might produce better results if you are not searching them together as a phrase.
For example: toxic chemicals
While it is true this could be searched as the phrase: "toxic chemicals"
It is also true that some authors might write about "chemicals that are toxic", and you would miss out on these results if you were to force the phrase search using quotations.