What does it mean when the hotel informs you that your free room has been received?

This is a voucher.

This is mainly a marketing tool for the hotel to provide discounts to guests so that they can book with them when booking rooms in the future, thereby increasing the number of times customers spend in the hotel.

This generally means that the hotel registers through a designated page and can get a free room opportunity, but the premise is that you must stay in the hotel for one night before you can get a free room opportunity.