The full name of any node prototype, up to the last "/" (forward slash) character, is the namespace. For example, in the prototype name
apis/airtable/Record fetcher, the namespace is "apis/airtable", and the name is "Record fetcter".
Namespace in a prototype's preview
Each forward slash in the full prototype name establishes a namespace, so in the previous example, the namespace "apis/airtable" belongs to the namespace "api".
Creating a namespace is as simple as creating a node with the namespace in the name.
You may explore the namespace structure of the Repo in the Repo reference section of the documentation.