Network segmentation is the practice of dividing a network into smaller, separate zones, often using virtual LANs (VLANs) and firewall rules. Devices in one zone can only reach other zones where it is specifically allowed.
A retail site might have separate zones for payment terminals and POS, back-office computers, security cameras, staff devices and guest Wi-Fi. Traffic between them passes through a firewall that enforces the rules.
The benefit is containment: if a guest device or a camera is compromised, the attacker cannot easily reach card payment systems. It also helps meet PCI DSS requirements to isolate cardholder data.
Segmentation needs planning and documentation, and the rules must be reviewed when new devices are added. A managed provider can design and maintain it so it does not become a source of outages.