The SourceRabbit 4-Axis CNC Motherboard uses the USB port available on all modern computers to download and send data.
In order to connect the SourceRabbit 4-Axis CNC Motherboard to your PC you will most likely need to install the necessary drivers for the CP210x chipset.
The drivers ar available for download at https://www.silabs.com/developers/usb-to-uart-bridge-vcp-drivers