Meta description
The meta description is the page summary proposed to search engines for display in results. It does not influence ranking but affects click-through rate, which itself eventually feeds into ranking signals.
In practice
Around 155 characters to create interest without overselling: a precise promise, one supporting figure, a measured call to action. Google frequently substitutes its own extract from the page if it judges that more relevant to the query. Writing the description is still worthwhile; relying on the engine to improvise is not.