In more detail
The title comes from the <title> element and the description from <meta name="description">. Aim for titles of about 30 to 60 characters and descriptions of 70 to 160. Search engines truncate by pixel width (about 600 px for desktop titles), so a title of wide letters can be cut early. They may also rewrite either one, especially when the description does not match the query. The description is not a direct ranking factor, but it strongly affects whether people click.
Why it matters
A missing or duplicated description makes the search engine pull a random sentence from the page, and a truncated title can hide the words a searcher was looking for. Each page should have its own.
Example
<title>Subnet Calculator – Hosts, Ranges and Masks</title>
<meta name="description" content="Enter an IP address and prefix to get the network, broadcast address, usable host range and mask. Works for any IPv4 subnet.">
The title is 43 characters and the description 124, both inside the usual limits. Preview the result with the SERP Snippet Preview.
Common confusion
Open Graph tags (og:title, og:description) control previews on social networks, not search results; a page usually needs both.