It's an "optocoupler" relay module operating at 12 Volt.
Does this mean that 12 V is galvanic isolated from the arduino, that should control the relays? My first impression was: I am missing a separate ground for the relay control and for the arduino!
However, after some study I discovered that you CAN operate the board with galvanic isolated 12 V and the arduino's 5 V.
How? Don't use the straps!!
So don't connect the 4 middle pins with straps to Ground or to VCC of the relay module, but connect these 4 pins to the arduino's ground!
Now the 12V and the arduino's 5V are galvanic isolated! See photo!