In practice
Write for the click, not for the ranking: one concrete promise in about 150 characters, matching what the page actually delivers.
Engines rewrite it whenever the page offers a better match for the query, so treat a good description as the default rather than a guarantee.
Why it matters
It is not a ranking factor, but it moves click-through rate — which changes traffic at an unchanged position.
A duplicated or missing description across a template is one of the easiest large-scale wins on any site.
Related terms