What the site list is
The site list brings all of a project's sites together in one place. Each site appears as a row showing its name, ID, type and region, so you can take in the whole project at a glance. Sites that belong to a larger site (sub-sites) tuck underneath their parent — select the arrow on a parent row to expand them.
You manage your sites from the site list in Project Settings: it's where you add sites, edit or delete them, merge duplicates, and review sites created in the field.