The source code of Barrelfish is documented using Doxygen.
This article assumes you have run though the steps in Getting Started, up through generating a Makefile.
Requirements
Doxygen (packet doxygen on Ubuntu or Debian)
Compiling
Change into the build directory of the Barrelfish source
cd barrelfish/build
and run
make doxygen
After the generation finished, open the following file in your favourite browser to access the documentation.
barrelfish/build/html/index.html