In a General Investment Account (GIA), you pay taxes on money you make from investments.
This includes taxes on dividends (profits from owning shares) and Capital Gains Tax when you sell your shares for a profit.
If you have investments in US company stocks, they deduct 15% from the dividends before you get them.
In the UK, you get an annual allowance of £2,000 for dividend income and £12,300 for profit when selling shares.
If you make more than these amounts, the amount of tax you pay depends on your income level, and the numbers might change over time.