What are the intensities from gazebo_ros_laser plugin

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The gazebo_ros_laser plugin sends sensor_msgs/LaserScan messages, that contain list of floats called intensities.

sensor_msgs/LaserScan:

std_msgs/Header header
  uint32 seq
  time stamp
  string frame_id
float32 angle_min
float32 angle_max
float32 angle_increment
float32 time_increment
float32 scan_time
float32 range_min
float32 range_max
float32[] ranges
float32[] intensities

Inside the the gazebo_ros_laser plugin source file, the list is being filled with following code.

std::copy(_msg->scan().intensities().begin(),
            _msg->scan().intensities().end(),
laser_msg.intensities.begin());

I understand the ranges list contains the distance to closest obstacle intercepted by the ray, but what does physically the intensity values represent?