Having taken apart several Chinese USB mains adaptors I can confirm that cheap Chinese electronics and mains power are not always a good match. Their adherence to EU safety standards can be abysmal, any CE mark is often just that - a mark that means nothing. However that one doesn't look too bad, looking at the pictures there is proper isolation of high and low voltages with just an opto-isolator bridging them.
If you have " limited experience and knowledge in electricity safety" you should probably let someone who has hook it up for you. At a minimum put in a box so it cannot be touched and very importantly put a very low current mains rated fuse in the mains input side. For that unit a slow blow 50mA one looks about right. A quick blow one would need a higher rating as those switch mode PSUs can take quite a surge on switch on. I guess you are in Greece so I don't know how your country does electrical things but don't just connect it to a mains outlet without fuse protection