Gift Aid is a UK tax scheme. When you donate and tick the Gift Aid box, the charity can claim basic-rate tax back from HMRC on your donation.
You must have paid at least as much tax as the charity claims. Gift Aid does not cost you extra — but you should not claim it if you have not paid sufficient tax.
CharityCompare scores do not depend on whether you use Gift Aid — but it is worth enabling on donations to well-run charities you trust.