If you are older than 28 years old, you might already know what a COM port looks like. Yes, COM port stands for Communication port. It was used widely to connect external device to the computer in the old days around 2005, such as: modem, mouse, even printer or transfer data between 2 PCs. Yes, COM port was understood as serial port.
Most cheap PL2303HX USB to UART adaptors from ebay usually don’t have DTR pin to upload sketch easier to Arduino Mini Pro.
This post will show you how to mod this little adaptor to play nice with arduino mini pro. It should take about 10 minutes and require good solder skill.