Single board computers are little computers no much bigger than a credit card, that can run a complete operating system with complex applications, and often give you access to regular and low-level peripherals. The most well known of these devices is the Raspberry pi.
In order to simplify terminology in this documentation we often refer to SBCs with the colloquial name: Microcomputer.
In the Chordata system an SBC acts as the main processing unit on the Host side. The main software running inside the microcomputer is called Notochord, so by extension we sometimes refer informally to the whole microcomputer as the notochord