Maximum load capacity as stated on the head refer to the maximum weight of the camera and lens combination. If say you have a camera with a 50mm len weights 1.1kg and the maximum capacity of the headt is 1.5kg, that means the head is capable to hold the weight of the camera. But if you also have a lens like a 70-200mm zoom....and you switch that with your 50mm lens on the camera, now your camera/lens now might be about 2.4kg, That means it exceed the capacity of the head to handle the weight. That does not mean the head will snap off and drop your camera to the floor. Maximum capacity in my view is more about stability issue of especially those tension spring ball head or those using a single screw system to hold the camera position pivoted over the ball surface. The extra weight of the camera/lens might slowly tilt off your preferred camera angle as it is beyond the strength of the head to hold the heavier camera/lens in the position and tilt you set for your camera. This is further made worst when your lens is heavier then your camera body..so there is already a lope-sided weight situation on top of that...so getting a bigger head will help alot in that case.
As for maximum weight of the tripod, it is similar to the above. In this case, the amount of stress and stability of the tripod to hold a given weight on top of it. But now, this weight is not only about the camera and lens BUT the weight of the tripod head too. As you might know, some tripod heads can be pretty weighty too. So you have to factor what kind of lens you have or intend to buy to make a pretty sensible choice of what tripod and head to get. Too heavy a weight on your tripod it will stress the joints of the tripod extensions and it can also slightly bend the legs overtime. That is why you see shops selling varying length and thickness of tripod legs...etc. The heavier your gears are, the more stronger your tripod or head will have to be.
That's about the size of it!
