That is how many calories you burn per kilogram of body weight for every kilometre you walk (or jog or run, it doesn’t matter – although, some caloric calculators show more for running than walking).
(In Imperial units that is 0.6 calories per lb of body weight per mile walked.)
While there are many sites that will tell you approximate calories burned for various activities (snow-shoeing, dancing, playing darts, swimming, etc), the problem with those is that the level of perceived exertion is subjective.
Estimating calories burned based on time spent at an activity is extremely subjective. A brisk 30 minute walk is subjective – my notion of a brisk walk is likely different from yours.
However, a 3km walk is objective. It doesn’t matter if it takes you 30 minutes or 90 minutes – the distance covered is the same and the level of exertion is the same.
It makes it much easier to visualize how far those calories you eat are going. For example, a typical 50g chocolate bar has about 300 calories. To burn those calories off, a person weighing 100kg would have to walk about 3.75km. A person weighing 50kg would have to walk 7.5km to burn off those calories.
(In Imperial, a 1-3/4 oz chocolate bar has about 300 calories. A person weighing 220lbs would have to walk about 2-1/4 miles to burn it off; a person weighing 110lbs would have to walk about 4-1/2 miles to burn off those calories).
It doesn’t really matter if you walk, jog or run all that matters is the distance you cover. If you run, you can cover the distance faster (at the expense of your knees).