You specify a phrase on most search engines by placing it in double quotes. Because they are unique identifiers , phrases are very useful at narrowing down the search results, since the search engine will only return pages that contain that exact, specific phrase. For example, some of the best phrases are full names. While there may be many people named Winston, or Spencer, or Churchill, there is only one famous person named Winston Spencer Churchill.
Song lyrics are good phrases to search for.
The best phrases are specific enough to get the pages you want, while filtering out pages you don't want. At the same time, they should be common enough to actually be found.