A host controller is a separate programmable microcontroller (typically) designed to control the graphical user interface and process user inputs.
A host controller is typically used when the display module is configured in Serial (SPE) or ViSi Genie environments. It is also possible to create a Designer or ViSi project that are designed to work with a host controller with a custom protocol.
Host controllers can be useful if there is a system which has a lot of GPIO or external device interaction, so it can be dedicated to handling those devices, and pass information to and from the display which is relevant.
Host controllers can also extend to larger devices, not just microcontrollers, but anything that has a Serial UART basically, which can pass information to and/or from a 4D display module.
Host controllers don't always need to be programmable either, such as if there is an older system and a display needs to be replaced on it, as long as the old display signals can be sent over a Serial UART, then a custom protocol can be written on the 4D Display side using Designer, ViSi or even ViSi-Genie PRO, to interpret this host information and display required information on the 4D display.