Pins 0 and 1 are used for serial communication over the USB port. That means uploading to your board and also Serial.print(), Serial.read(), etc. You also have the option of using those pins for other purposes but you need to be aware that could interfere with their use for Serial communication. It's best to just leave those pins alone if possible.
Pins 14-19 on the Mega do have the capability of being used as additional hardware serial ports but they're not connected to the USB interface so you are free to use those pins as you like.