ROS Arch linux wiki question

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Hi

I have a question about the following peice of code in http://wiki.ros.org/indigo/Installation/Arch

# ROS
indigo() {
  source /opt/ros/indigo/setup.bash
  export ROS_PACKAGE_PATH=/path/to/your/your/package/path:$ROS_PACKAGE_PATH
  export PYTHONPATH=/opt/ros/indigo/lib/python2.7/site-packages:$PYTHONPATH
  export PKG_CONFIG_PATH="/opt/ros/indigo/lib/pkgconfig:$PKG_CONFIG_PATH"
  alias python=/usr/bin/python2

  # Gazebo
  source /usr/share/gazebo/setup.sh
}

In this i did not understand the environment variable ROS_PACKAGE_PATH. Is is needed ? What is the role of it ?