Boundary has a rich domain model
consisting of resources that represent
and the means to tie them together.
This section contains information about the various resources within the domain
and how they relate to each other.
A role is a resource
that contains a collection of permissions
which are granted to any principal assigned to the role.
A session is a set of related connections between a user and a host.
A session may include a set of credentials
which define the permissions granted to the user on the host for the duration
of the session.
When getting started with Boundary,
the first resource to look at should probably be Scopes.
Every other resources is either contained within a scope,
or contained within another resource
that is itself contained within a scope.
That page can help you understand the structure of resources within Boundary.