a kilojoule or kJ is a unit of energy, as is a calorie. A kJ is .239 kilocalories. It happens that the body is 23%-25% efficient in turning food energy into work done. Thus kilocalories burned is about equal to kilojoules. When I look at my Strava results, I figure that kJ is my calories burned or at least way, way closer than any other method of measuring them. Strava's estimate of kJ can be thrown off by headwinds or tailwinds, but if your personal and bike data entered is correct, it will be very close. It derives this number from the equations of motion, not from a heart rate monitor or other inaccurate device.