To buy a US share, your pounds have to become dollars first. The spot rate is the wholesale exchange rate at that moment, the one quoted in the news. Your broker converts at a worse rate, and the difference is its FX fee: sometimes a stated percentage, sometimes a conversion spread built into the rate it offers, often both. Rates vary widely between brokers, and the cost often shows up only in the rate on your contract note, not as a separate line.
It's charged every time money crosses currencies: when you buy, when you sell and bring the proceeds home, and on every dividend converted to pounds. At 0.75% each way, a round trip costs about 1.5% before any dealing charge. Some accounts let you hold dollars, which changes when you convert rather than whether. The app values foreign holdings and dividends at a daily market rate with no conversion fee, so what you'd get back in pounds after selling would be a little less than it shows.
An illustrative rate, before any price move or dealing charge. Your broker publishes its own in its list of charges.
Educational content only. It is not personal financial advice.