The following is my opinion as I do not have any documentation to properly check it, only what I can see from the picture and description from a seller that did not know what he was saying. Personally I would send it back, the relay coils appear to NOT be isolated from the micro and can be a source of noise, transients and other confusing problems.
Without a schematic I can only guess. There are 5 transistors on the board, I assume they drive the relays but that is only a guess. They indicate a blue light which appears to the the converse of the input, if that is correct the transistor would probably do the inversion from the input and drive the LED. The inputs would be connected directly to the base of the relay driver transistor or to a pull up from the 12 Volt supply. When they say port they typically mean an Input or Output pin of the microprocessor. On the Arduino they are the "D" digital pins. They state" Relay low-level pull-in, the module contains current-limiting resistor, possibly to the base of the transistor, the low-level can be DC-, the microcontroller ground. You would set microcontroller I / O port to low to turn off the relay, again guessing as I have no circuit." You can buy a properly isolated eight channel unit from china on ebay for several dollars less then what you spent on this one.
I think that is there way of telling you you need a LOW on the port ("D") pin. I would be cautious as if the ground connection between your Arduino and this board opens very weird things can happen and possibly damage the Arduino.
Best guess the circuit is a transistor with the emitter connected to -DC, The collector to the relay coil and the other coil connection connected to DC+. If this is correct a high on the arduino port ("D") pin would turn the relay on.
Good Luck, and Have Fun!