What the number means
The spread is the gap between bid and ask. You buy at the ask and sell at the bid, so every position opens slightly negative by exactly that amount multiplied by pip value and size.
Why it moves
Spreads track liquidity. They are tightest during the London–New York overlap, wider in thin Asian hours, and widest around rollover, weekend reopen and high-impact data releases.
Measuring your real cost
Multiply typical spread by pip value by monthly lot volume. Scalpers often find spread is their largest single expense; swing traders usually find swap matters more.