Some basic doubts related to PX4/Gazebo/MAVROS/Mavlink

I am trying to setup PX4 SITL using ROS/Gazebo as the simulation environment. I am following the instructions given in the official PX4 documentation ( I had some doubts about the setup.

  1. What exactly does the command make px4_sitl_default gazebo do ?
  2. While launching the MAVROS node to communicate with PX4 Firmware we launch the following launch file using : roslaunch mavros px4.launch fcu_url:="udp://:14550@". May I know what exactly is the fcu_urland what does udp://:14550@ stand for ?
    I’ll be very grateful to get these answers,