It is an option to enable the machine to "accept" an operating system (OS) via a server, rather than having the OS present on the machines hard drive. It can also be used to load a "Corporate approved" system, to a local machine. What is meant by this is large corporations sometimes significantly modify a standard Windows installation, to suit the corporate environment and specifically, to work well with sometimes bespoke software. In this case, it is easier to have a standard image to load via a network, rather than modify each machine individually.
It is relatively complex to set-up and requires specific network cards with certain capabilities (PXE compliant boot) and a DHCP or BOOTP server on the network in order to work.
If you are referring to a home machine, disable the option in the BIOS, as it slows booting considerably.