If there is the device buffer size too small on using DirectX Port, the Overload sign will flash. Using the ASIO Port it will flash if the ASIO System can't process the data. Increase the ASIO Buffer size in the ASIO Dialogs of your soundcard.

The CPU indicator displays the CPU power used by the DSP functions. This value represents the real audio CPU usage not the CPU processing power consumed by other applications. Note that 100% audio usage will crash the system because no power remains for controlling other devices like the display, keyboard or mouse! Before that happens, the DSP will "put the brakes on" and trash some samples. Noise will appear in the output. 

The MIDI In and MIDI Out indicator flashes on incoming and outgoing MIDI-Events. Use MIDI Setup dialog to define the MIDI Controller and Key-Groups.