Metadata means “data about data”. On the web, metadata usually means “data about webpages”, such as what the page is called, and who it was created by.
Examples of metadata
Title – every page should define a title, which is shown in browsers, search engines and social networks
Description – a page can optionally define a short description, which is visible on Google and Facebook below the title
Image – (more accurately, an OG image) a page can define an image to represent it when shared elsewhere, e.g. in a Facebook feed
A common false assumption is that metadata means meta tags. This is not the case. Many types of valuable metadata, such as titles, are marked up in different ways.
Why metadata matters
Metadata provides crucial data that helps third parties understand and present your webpages.
In particular, the effective use of metadata makes it more likely your pages will be presented effectively in search engines and social networks, and therefore be more likely to be visited.
Further reading
Page title – Silktide
Meta tags that Google understands – Google
Complete list of meta tags – GitHub