Mohammed Manna

Hello, Whilst completing my SA returns for 2023-2024 - I noticed this breakdown. I have already paid the tax I owed due to my Employment. On top of that, I declared additional income. And on the final screen I saw this: Total amount due for 2023-24 - £X Plus First payment on account for 2024-25 - £Y Total to be added to Self Assessment account due by 31 January 2025 - £X+Y What is First Payment on account? Also - I think the breakdown of calculation correctly showed how much tax I owe, besides what I'd paid. And this is the first time such payment is being asked for as I had done SA returns a few times already. Can you explain what this is? The website doesn't explain...