Trip planning is planning your trip so that you are not running multiple errands separately. This is especially important when you are driving.
For example if you need to say go to the post office, pick up some milk and drop your kid at an extra curricular activity - then you plan out your trip. Not go and pick up the milk in the morning, return home and then after lunch go to the post office and return home and then late afternoon deliver your kid to his karate practise. Instead you leave a little early for the karate pick up the milk and go to the post office on the way to the karate lesson. This saves huge amounts of gas, energy and reduces your carbon emissions.
Since I am using karate lessons - you can guess my son does karate. Instead of dropping him off and returning home only to pick him up later. I either stay and watch the practise or I take my little ones and go for a walk and do some 'window shopping' again saving some milage, gas, and carbon emissions.
On a side note I was impressed by my husbands green action. I came downstairs after settling our little one down for the night and noticed our garbage can in the middle of the driveway with the lid open in the pouring rain. Puzzled by this I ask my husband what he was doing. "I am washing it out. I figured instead of using the garden hose to rinse it out. I would collect some rain water in it instead and use that to rinse it out." was his confident response. WOW was my reaction - this green thing does rub off on him in little ways. Way to go hubby for thinking green.