What a requote is
A requote is the broker saying: 'the price you asked for is gone, here is a new one — do you still want it?' It happens on Instant Execution accounts, where you request a specific price and the broker either fills it or offers an updated one.
It is not the same as slippage. With a requote you get a chance to accept or decline; with slippage you are simply filled at a slightly different price.
Common causes
Fast markets where price moves between your click and the broker receiving it. News releases and the market open. A slow connection adding delay. And, in some cases, an execution model that requotes more than a competitive broker would.
How to reduce it
Set a 'maximum deviation' in the order window so small price moves are accepted automatically instead of triggering a requote. Consider a Market Execution account (common on MT5), which fills at the best available price rather than requoting.
Avoid firing market orders in the first seconds of major news. Improve your connection so there is less lag between click and fill.
How to prevent it
Choose an execution type that matches your style, allow a sensible deviation, and trade around news rather than into it. Pending orders can also sidestep the click-to-fill delay.
Broker's fault or yours?
Occasional requotes in fast markets are normal and fair. Frequent requotes in calm conditions, or requotes that always seem to move against you, suggest an execution model worth questioning — compare the same trading on another broker if you can.