A 'sliding scale' is when something which is set or fixed. is adjusted, shifted up or down in value/magnitude, according to how some other factor varies.
Can you give the context from the passage you are reading - sounds like it's to do with grading test paper marks and difficulty of one test paper compared to another test paper.
A new wonder drug that means people who are fat get slim, and those who are slim never get fat. The catch: it costs £10 a pill. The government sees this as an opportunity, essential to fighting obesity, but can hardly afford the cost to provide this free to the whole nation. At the same time, how can someone out of work, and so morbidly obese that they can't work, afford this, if they have to take one pill every work for the treatment to be effective?
If you earn over £X pounds, you pay the full £10 per pill you pop.
Earn between £Y and £X pounds, and you pay £5
Less than £Y, free.
In simple words, the cost(first variable) is adjusted according to your ability to pay(second variable).
Student or Learner