DHCP
The service that automatically assigns IP addresses and network settings to devices when they connect.
Without it, every device would need manual configuration. DHCP hands out an address, gateway, and DNS servers from a defined pool, with a lease that expires and renews.
Two things commonly break: pools sized too small for the number of devices, and a second unauthorized DHCP server appearing on the network, which hands out wrong settings and takes users offline unpredictably.
Where this comes up
DHCP sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.