Glossary
Meta description
A meta description is the short summary a search engine may show underneath your page title in the results.
It is not a ranking factor and Google frequently rewrites it. It is still worth writing, because when it is used it is doing the work of persuading somebody to click.
Why it matters to you
Write it for a person deciding between results, not for a search engine. Around 140 to 155 characters is the space you actually get before it is cut off.
An example
Of 300 local sites we measured, 27% had no meta description at all, which leaves the search engine to assemble one from whatever text it finds first.
Where this comes up: search engine optimisation .