Apache (the service is usually named httpd) receives browser requests and serves your websites, passing PHP to a handler such as PHP-FPM. Panel servers define each site as a virtual host.
Two things deserve monitoring: that the service is up (when Apache dies, every site dies), and that it has free workers — Apache handles concurrency with a worker pool capped by MaxRequestWorkers, and when all workers are busy new visitors wait or get errors.
Frequently asked questions
What is a virtual host?
A configuration block mapping a domain name to its files, logs and settings — one server hosting many sites side by side.
Why do sites hang when CPU is low?
Often Apache worker saturation: every worker slot is busy with slow requests, so new ones queue. Watching busy workers versus the limit reveals it.