Technical
Meta description
Meta description is the `<meta name='description'>` tag in a page's head that supplies a summary a search engine may show beneath the title. It is a suggestion, not a command.
How it is measured
View source and read the `content` attribute. Engines truncate by pixel width, roughly 150 to 160 characters on desktop and fewer on mobile. Crawlers flag tags that are missing, duplicated, or too long.
Search Console's performance report gives impressions and click-through per page, so you can compare a page before and after a rewrite. Google swaps in text from the page itself whenever it thinks that fits the query better.
Worked example
A WordPress recipe site has 340 posts sharing one plugin default: the first 150 characters of each post. After rewriting the 20 posts with the most impressions, Smoky Lentil Soup goes from 1.9% to 3.1% click-through over four weeks at a similar position.
Google still rewrote the snippet for about half of that post's queries, using a line from the ingredients list instead.
How it differs
A meta description is the summary for a search results page. An Open Graph description is the summary for a shared link preview, read by a chat app or social site. One sentence can serve both, but they are two tags fetched by two different crawlers.
Common errors
Leaving the plugin default. Using one description across the whole site. Stuffing keywords. Running past the cut-off so the ending vanishes. Expecting it to move rankings directly. Putting a double quote inside the content attribute and breaking the HTML.
In practice
Export titles and descriptions with a crawler, fix blanks and duplicates first, then rewrite the pages with the most impressions. Answer one searcher's question in the first 110 characters.