What is a good metadata description ?

The meta description is the text that appears below the title of the page on search engine result pages. This text can be entirely customized by the owner of a site, but Google reserves the ability to change it and use content from the page in the meta description if it judges the original description non-descriptive, inappropriate or irrelevant to the actual content of the page.

A meta description should not be longer than 160 characters, spaces incluced. It should describe what the page is about and include at least one keyword used in the corresponding title of the page.