Metadata is the information that shows up when your storefront appears in a search engine or a browser tab, the title, the short description, and the small icon (favicon) next to it. When you fill in your metadata in the portal, that's what people see when they find or share your storefront.
It helps in two ways: it supports your search presence (SEO), and it makes your storefront look more polished and credible, since a clean title, description, and icon signal a professional, established business.
Metadata settings are available on the Pro plan.