A meta description is a short summary of a page, set in its HTML, that search engines may show under the title in search results.
Search engines often show roughly the first 150–160 characters and sometimes write their own snippet instead, especially when another part of the page matches the search better.
It isn't a direct ranking factor, but a clear one makes people more likely to click your result.
Why it matters for a launch
For a new site with few rankings, every click counts. A description that answers the searcher's question improves clicks from the impressions you do get.