+ FreeRTOS/source contains the FreeRTOS real time kernel source code.

+ FreeRTOS/demo contains a pre-configured demo project for every official 
  FreeRTOS port.
+ See http://www.freertos.org/a00017.html for full details of the FreeRTOS
  directory structure and information on locating the files you require.

  + FreeRTOS-Plus contains additional FreeRTOS components and third party 
  complementary products.  THESE ARE LICENSED SEPARATELY FROM FreeRTOS although
  all contain open source options.  See the license files in each respective
  directory for information.
+ FreeRTOS-Plus/Demo contains pre-configured demo projects for the FreeRTOS-Plus 
  components.  Most demo projects run in a Windows environment using the 
  FreeRTOS windows simulator.  These are documented on the FreeRTOS web site 

Further readme files are contains in sub-directories as appropriate.
The easiest way to use FreeRTOS is to start with one of the pre-configured demo 
application projects (found in the FreeRTOS/Demo directory).  That way you will
have the correct FreeRTOS source files included, and the correct include paths
configured.  Once a demo application is building and executing you can remove
the demo application file, and start to add in your own application source

See also -