Phrase searching is a very effective way of narrowing your search to documents on your topic. By putting quotation marks around two or more words, you force the database or search engine to find your words together in the document instead of separately. For example, if you search for food security without quotation marks, you will receive results which have the word food in them and the word security in them, but not necessarily together. If you search "Food Security", you will only receive results with that exact phrase.
To see this in action, try a search in your favourite search engine for the words food security with no quotation marks. Now try a search for "food security". How do the results differ?