Metadata works behind the scenes on your website. SEO metadata is what appears on search engine result pages (SERP) when a website comes up for a particular query.
Basically, it enables your website to become relevant in a search by people who are searching for what you offer.
While this article covers a more advanced topic, it's worth taking a look to appreciate this vital part of SEO.
You won't need to apply these concepts yourself (that's what we're here for) but discussing metadata with your SEO guru can improve your SEO campaign.
