Serial interface is a simple way to communicate between your PC and your device, including an FPGA. An Arduino board is used as a USB-to-serial converter, sending a word 8’b1010_1010 and 8’b1100_1100 alternatively.
A simple logic in an FPGA generates a sampling pulse for the serial data. Note that the first “0” is a start bit, which does not have to be sampled.