A demo application is a project that runs a specific port on a specific hardware platform.
The microcontroller architecture specific part of a FreeRTOS port is called the port layer. Each port layer is either 'officially supported' or 'contributed'. This page explains the differences between the two.
A demo application is a pre-configured project that targets a specific development board and uses a specific tool chain. It is intended to allow ‘out of the box’ usage even for new users, and includes many examples of how the FreeRTOS API can be used. Some official ports include multiple such demo projects to provide users a choice of both hardware and build environments.