IPMI (Intelligent Platform Management Interface) is a strategy whereby computer hardware can provide a managements interface independent of operating system, cpu or bios (or uefi). See https://en.wikipedia.org/wiki/Intelligent_Platform_Management_Interface for a full definition.

Basically, it allows you to turn on/off the system, make a connection via the network to the screen, keyboard and mouse, and monitor the actual hardware of the system.

Management can be performed via open source software ipmitool, and generally through a web interface provided by many machines equipped with IPMI.