Why do some IC pins have an asterisk behind them?
I assume you mean Arduino Uno or Nano with Atmega328p, specify it next time
The best way to figure it out is to look into the datasheet. Of course, if you only have experience with Arduino, it's actually pretty difficult and scary, there are a lot of words you probably never saw or only just saw before. I will try to make a short summary of the important points.
In this case, PD0 and PD1 are actually the serial port pins of arduino. You program arduino through them if you just do it like you always do - 1 cable to pc and nothing else. But you can use those pins for something else, bear in mind tho, you'll need to program arduino with external programmer then. The bootloader starts the serial port for you so you can program the thing comfortably. You need to change bootloader to use them as digital pins, which, unless you have a programmer around just in case something goes wrong and you need to restore the bootloader, I don't recommend you to do.
But if you DO have a programmer around - another arduino as ISP will do - give it a shot, I mean, it's not all that difficult, there's plently of manuals for atmega328(p) bootloaders and stuff. It's like uploading a small program, nothing more, just with a dedicated programmer.