A Site is a real location with an address and Assets. These are currently Chargers and Meters.
It's important to always link your Assets to a Site, because some things are managed on a Site-level, such as:
Load management (load, solar, sharing, etc)
Smart Charging (charging on energy prices)
Permissions (who can view and edit)
Ownership
Charge cards and access
How to create a Site?
Web Portal
Go to Sites
On the top right, click "New Site"
Give the Site a name (can be anything)
Fill in the address. It's important to get the correct Country if you plan on using Smart Charging.