Hello @diwz Asked great from beginners perspective.
Google crawlers and robots use meta-description and tags for search engine snippets but sometimes they do not show because Google is not bound but they also show relevant info that what user is demanding.
Meta description and Meta title both are essential in Seo on-page factors,
We use script tags to represent meta tags behind the website and it is readable for humans and robots. Meta description tag example:
<meta name="description" content="Your description here"/>
Meta description recommended length 150 characters include space.
Meta title tag example:
<title>Your title test here</title>
Recommended length 60 characters.
Third was meta keywords but now Google doesn't consider meta keywords in search ranking.
Hope that will help you!