A Capability blob is a signed piece of data which grants permissions to other identities.

A permissions are the "right to do something" in the Hypermedia network.

You have full permissions over your space, which is defined by your public identity/key. From there, you may grant additional permissions with Capabilities. Or, others may grant Capabilities to you.

In the future, you will be able to grant more Capabilities if you are given a sort of "admin" Capability. But this is not yet implemented.