Vital functions are the basic processes that keep a living system alive, such as breathing and heartbeat regulation. In cyber security writing, the phrase is sometimes used metaphorically for the most essential services in a digital environment: authentication, identity checks, network connectivity, logging, and other processes that must keep working for the system to remain usable.
This matters because attackers often aim at vital functions first. Disabling a directory service, overwhelming a network gateway, or corrupting a control process can stop normal operations even if the rest of the environment is intact. Defenders protect these functions with redundancy, segmentation, monitoring, failover, and recovery plans. The goal is to keep essential services available and to ensure the system can degrade safely instead of failing all at once.