"I am confused..."

No doubt. You are looking at code likely targeted for running on a PC. You'll need to change the code dealing with the PC's I/O port addresses to the UART-related 8051 SFR's, unless of course your 8051 board also has an external 8250-compatible UART, in which case you'll have to deal with it in a manner appropriate for how it's connected to your 8051 (e.g., direct port I/O versus external memory mapped I/O).