Imagine you have a piggy bank where you keep your money. Every time you get some money, you put it in the piggy bank. One day, your mom tells you that you have to give a little bit of your money to the government, so they can use it to make sure everyone gets things like schools and hospitals. This is called paying taxes.
In the UK, the government has set up different levels of taxes you have to pay, depending on how much money you make. The starting rate of UK income tax is like the first level of taxes you have to pay. If you make less than a certain amount of money, you don't have to pay any income tax at all. This amount changes each year and is called the personal allowance.
Currently, the personal allowance in the UK is £12,570 per year. This means that if you make less than £12,570, you don't have to pay any income tax. If you make more than that, you have to pay a starting rate of 20% on the amount you earn over the personal allowance.
Let's say you make £15,000 per year. Since the personal allowance is £12,570, you have to pay income tax on the extra £2,430 you earned. The starting rate of income tax is 20%, so you would have to pay 20% of £2,430, which is £486. This means you would have to give £486 of your £15,000 income to the government as income tax.
So, the starting rate of UK income tax is the first level of taxes you have to pay if you make more than the personal allowance. It's like giving a little bit of your money to the government to help pay for things like schools and hospitals, just like when you put some of your money in the piggy bank to save up for something you want.