"IPC" refers to Inter-Process Communication, a mechanism that allows different programs or processes to share data and communicate with each other, while "Server Units" often refers to hardware or software that provides services to other applications or devices.
Interprocess communication (IPC) is a process that allows different processes of a computer system to share information. IPC lets different programs run in parallel, share data, and communicate with each other.
IPC is very important to the design process for microkernels and nanokernels, which reduce the number of functionalities provided by the kernel. Those functionalities are then obtained by communicating with servers via IPC, leading to a large increase in communication when compared to a regular monolithic kernel.