The perfect example of this is traffic. Traffic is a pain but there is nothing you can do about it. If there is nothing you can do about it then there is no reason to get frustrated and angry. Getting worked up will not make the traffic better and will only make you stressed out.
So instead of getting road rage, control the things that you know you can control such as leaving yourself enough time for travel expecting traffic. Think what is the longest time it takes for this trip, instead of what was the quickest time I ever made this trip.
Control what you can control. Don't focus on what you can't control.
Now we can start to apply this concept to all areas of our life. In the gym, at work, at home, and trips to Wal-Mart.