May be, can someone advise me one thing: I want to order ticket for the train Berlin-Warsaw for 23 July and I see on the web db.de one special offer for this train (29 euro).
Unfortunetely I can't understand what does it mean this special offer?
That discount is just an advanced reservation discount - many train routes will have discounted tickets if you are one of the first certain number of people to purchase it. If you wait too much longer before purchasing it, you may have to pay the full price.
Is it for everyone? Not only for german citizens?
That is correct, for anyone who books the trip.