Limb have many uses. A limb is a piece of an object that can be controlled either seperately from and object or with the "parent" or main object. If you make a huge level and make it out of many limbs you can use the perform checklist for object limbs to get data about each limb. A limb can be a turrent on a tank, a door to a level if created with the level, or even a wheel on a car.
Constants are variables that once created and assigned values to you cannot change their values. This keep you from over-writing them accidently.
globals are variables that can be accessed from anywhere in you program without having to pass them to functions. all variables created are local by defaul and cannot be accessed from functions unless you pass them to it in the parameters of the function. So you use global to allow it to be used with a funtion without having to pass the value in everytime. dimed arrays are global by default.
Hope this helps.