What is the basic idea of perimeter security, and what draws the system boundary?

Distinguish an inside from an outside and regulate every crossing between the two. The system boundary is a network boundary, and network addresses and protocol types are what tell actors and actions apart.

"Perimeter" means circumference or border. A perimeter fence is the fence around a site, and a security perimeter is a security zone. The term moved into cyber security, where its first use was access control via the network.

The guiding thought is "only allow secure networks": inside is assumed good, outside bad, and firewalls and similar devices act as border control at the crossing points. Who you are and what you may do is decided by where your traffic comes from (source address) and what kind of traffic it is (protocol and port). Everything else about this model, both its strength and its weaknesses, follows from that one decision.