The meta description is set with <meta name="description" content="..."> inside <head>. Search engines may use it as the snippet shown beneath the page title in results, or they may rewrite it from on-page text if they think a different excerpt fits the user's query better.
Meta description is NOT a direct ranking signal -- Google has stated this explicitly multiple times. But a well-written description improves click-through rate from the SERP, which is a measurable downstream traffic lift. Aim for 150-160 characters: under truncates to "..." in mobile results, over gets cut by Google's snippet length limit.
Every page should have a unique meta description. Templated descriptions ("Buy {product} on our store") are worse than no description at all -- Google often rewrites them and the duplicate-content signal hurts. Write the description like ad copy: lead with the user benefit.