Sliding scale fees are fees that change depending on how much money you have. So if you have a lot of money, you'll be charged more, and if you have less, you'll be charged less. For example, You might have to pay a fee to use a service, but if you don't have a lot of money, you won't have to pay as much as someone who does have a lot of money. It makes things fairer, so everyone can have access to the services they need.