We've written before about the Gift Aid box when you make a donation, what it means, and - amongst other things - why anyone with income that takes them into higher rates of tax should include the donation on their tax return.
Ordinarily, you'd just include donations made in the year in question on your tax return. In fact, there's an alternative option, which is to "carry them back" and include them on your tax return for the previous year, if you've not yet submitted it. Generally, it's not worth the hassle - if you're donating on a regular basis and your income doesn't vary wildly year-by-year, who cares if a £10 a month donation saves you tax this year or next year? It's not worth the hassle of keeping track of what's been carried back and making sure that you don't claim it again next time, and waiting to file your return at the last minute so as to carry back as many donations as possible.
However, this year there are likely to be a lot of people who made unusually large donations in the early part of the tax year, for obvious reasons. If you donated a one-off £1,000 in mid-April, say, as a higher-rate taxpayer you're going to get £250 knocked off your tax bill. So the amounts involved start to become a bit more interesting, and you might well consider carrying the donation back and sticking it on your 2019-20 return to save the tax early.