Tax amount due split into two parts

I have just completed my self assessment and I can 'view my calculation'. I am surprised that the total amount shown is not shown as 'Total amount due for 2023-24', but it says: 'Total amount due for 2023-24' Plus 'First payment on account for 2024-25' and the sum of these two values is described as: 'Total to be added to Self Assessment account due by 31 January 2025' Can you please explain why the summary is presented this way? What is 'First payment on account for 2024-25'?

Hi,
Please have a look at the guidance at Understand your Self Assessment tax bill.
Thank you.