Instant execution means that orders placed by a trader can receive a different price, which is different from what they see in the trading platform. In this case, the trader receives Requote - a different price offer.
Sometimes, due to the rapid movement of prices in the market, for example, during the release of the important economic news, at the moment of issuing an order, the prices in the platform can change a lot and the trader receives a Requote. This indicates that the broker is using the Dealing Desk model, i.e. processes the trader's deals before execution.