Multi-client accounts (MCAs) in Google Merchant Center allow advertisers to easily:
Combine reporting on all data feeds across domains
Share products between accounts
Manage multiple domains with a single sign in
Access multiple AdWords accounts
Explanation:
Merchant Center multi-client accounts allow a single person or company to submit data feeds for multiple domains. Each client may have a distinct store name and URL. When you request a multi-client account, your current Google Merchant Center account will become a sub-account in your new multi-client account.
Read more here: https://support.google.com/merchants/answer/188487?hl=en