Security boundaries are often thought of as firewalls, IPS/IDS systems, or routers. Also, logical setups such as DMZs or other network constructs are often referred to as boundaries.
But in the modern world, where many companies support dynamic work models that allow you to bring your own device (BYOD) or are heavily using online services for their work, the real boundary is the identity.
Identities are often the target of hackers as they resell them or use them to steal information. To make the attacks as hard as possible, it's important to have a well-conceived and centralized identity management. The Azure Active Directory provides that and a lot more to support your security strategy and simplify complex matters such as monitoring of privileged accounts or authentication attacks.