Why it matters
Two results at the same position can have wildly different click rates because one's snippet makes a case and the other's is truncated boilerplate. It's ad copy you don't pay for, and most sites autogenerate it badly.
How it works
You set it per page (about 150-160 characters). Google rewrites it when it thinks it can do better, which happens more when descriptions are vague or duplicated. Specific, benefit-led summaries that match the query survive intact more often.
What to do about it
Write real descriptions for your twenty most important pages: what the page delivers, for whom, with a reason to click. Skip the keyword-stuffing: it reads as spam to humans and gets rewritten anyway.
