This reminder usually refers to
Additional fees for additional services you have not yet paid for, such as laundry, phone calls, liquors from the mini-fridge, et al.
But since the statement you are asking about is in the section about children and extra beds, it probably means: here are our rules about these things; we will charge you extra if you break them. Meaning if you try to stick eight adults in a room meant for two, or you have six children and not two, you will be expected to pay extra.
Edit for ColleenV's point:
Hi. I looked up a room for two nights in a hotel I am marginally familiar with. I see the webpage price. Near that is a 15% tax not included text. I went to actually reserve the room and it seems there are no "hidden fees," just the original price from the first page and the 15% tax added in:
>Room $458
TAX (15%) included $68.70
Today you'll pay $0
You'll pay at stay $526.70
Your booking includes:
- Free WiFi
- FREE cancellation
Total Price
$526.70
(for 2 guests and 2 nights)
So I think that covers it. The website looks pretty on the up and up. But before I gave them any Personal Information, I would double check.