A SharePoint Online Administrator manages the SharePoint Admin Center. The Administrator has the ability to create new Site Collections in a SharePoint Online Tenancy and can also Enable External Sharing. The Storage Quota for each site collection can also be managed. One could Assign a Storage Space anywhere between 100 MB- 1 TB.
A point to note that a SharePoint Online Administrator by default is not a Site collection Administrator. They need to add themselves as the Site Collection Admin by selecting a Site Collection and clicking on Owners on the Site Collection tab and then Manage Administrators.