Understanding it
Google builds the snippet from the page content, depending on the search. The meta description is only one possible source: it is used when it matches what the searcher wants.
A good description is specific to each page, sums up what is there and gives a concrete reason to click: a price, a timeline, a proof.
It does not affect ranking directly, but it affects the click-through rate, which decides real traffic.
A concrete example
Illustrative example: a generic description
All of an agency’s service pages share one description: “We support your digital projects.” Google ignores it and shows random bits of sentences.
Each page gets a description summarising its offer and main strength. Snippets now match the titles.
The common mistake
Copying the same description everywhere
An identical description on dozens of pages helps neither Google nor searchers tell them apart. Write one description per important page, and let Google pick the snippet on secondary pages.