when I created a Sharepoint site collection on the root. it comes out as
https://mydomain.sharepoint.com/**sites**
What are this Sites means?
Is it possible to create a site's collection without having Sites in the URL?
Sites/ is a Wildcard Managed Paths which specify that one or more site collections exist at a specified path!
There are two types of managed paths that you can create:
wildcard inclusion
allows you to append multiple site collections to the path that you specify. For example, if you add /engineering as a wildcard inclusion off of your root site of http://contoso, then you'll be able to create multiple site collections off of http://contoso/engineering.
explicit inclusion
allows you to create a single site collection with the specified address. For example, if you add /finance as an explicit inclusion off of your root site of http://contoso, then you'll be able to create a single site collection with the address http://contoso/finance.
For more details check Managed Paths in SharePoint
Is it possible to create a site's collection without having Sites in the URL ?
Unfortunately, you can't create/control Managed Paths
in SharePoint Online. but you can only manage it in SharePoint On-Premises