CFDs are a leverage product and can involve a significant risk of loss. Trading CFDs may not be suitable for all, therefore you should ensure that you understand the risks involved and take into account your individual circumstances.
Trader's Glossary

What is Maximum Deviation?

In MT4’s order execution window you can choose to set a maximum deviation pip-range for your orders. When the maximum deviation box is left unchecked your order will only be executed if it is available at the exact price you requested, if not the order will be rejected. Maximum deviation allows you to specify a pip-range that is acceptable to you in the event that your order cannot be filled at the requested price. In other words, checking the maximum deviation box and setting it to two pips indicates that you are willing to accept a price that deviates 2 pips above or below the requested price. In this way you can reduce the number of rejected orders, which is especially helpful at times of high market volatility.