Requotes happen on dealing-desk (market-maker) brokers when the broker's internal price has moved between the time you clicked and the time the order reached their server. The platform shows a dialog asking you to accept the new price or cancel. On fast markets, this can repeat several times in a row.
ECN and STP brokers do not requote — they either fill the order at the best available price in the liquidity pool or reject it entirely. If you see requotes often on your broker, that is usually a signal the broker is running a dealing-desk model with aggressive flow management.