If your personal tax bill exceeds £1,000 in a year, you will be required to make payments on account towards next year's tax bill, as well as pay your tax bill for this year.
So for example if your 2016/17 tax bill is £2,000, you will have to pay £3,000 in January 2018. This is £2,000 plus £1,000 (half of your expected 2017/18 tax bill). In July 2018 you will then have to pay the 2nd payment on account of £1,000.
The next year, if your 2017/18 is £2,000 again, you'll have already paid the bill through the payments on account, so will only be liable to pay £1,000 in January 2019, which is half of your expected bill for 2018/19. In July 19, you'll pay the 2nd payment on account of £1,000.
If your tax bill for 2017/18 is not £2,000 there will be an adjustment payment or refund to make, then if the bill is still over £1,000, there will be payments on account due in January and July 19 of half of the last tax bill submitted.