You make trades based upon the information available at the time. If your information says market X will move down and Y will move up - you want your trade com

You make trades based upon the information available at the time. If your information says market X will move down and Y will move up - you want your trade completed before that happens. Any changes between the time the decision is made and when the trades are executed are referred to as slip. Assuming you're original trade decision was correct this slip will be against you.