Rent percentage means the % of the original rent of the house. For example, let's say the house is 25,000 rent (which you can see the price on the website or in the cyclopedia)... 10% of 25,000 is 2,500. This is what you'll have to pay for the house aside from the bid on the house.
Here's a quote from the forums on Tibia:
Q: "On the settings it's says that will be possible to rent a house.. after the tournament end, all rents will be lost?
A: Yes, but rents will only be 10% of the original ones. Also, house auctions only last till the next server save."
Source- https://www.tibia.com/forum/?action=thread&postid=38695615#post38695615