A tenant is an isolated private namespace that holds all resources related to identity management, such as applications, users, groups, etc.
Your tenant URL would be <tenant_id>.crossid.io (e.g., "acme.crossid.io")
Typically each organization have a single tenant.
For b2b companies (such as SAAS), each customer of yours should have its own tenant, see b2b for more info.
If you don't have one, register one for free.