They're the identification numbers by which the computer refers to the various contols (listboxes, checkboxes, pushbuttons, etc.) when it needs to do something to them (like set their text, size, state, color, etc.)
In English, a handle is some kind of structure that you can grab and control an object with, like a door handle, the handle to your car door, the handle to your suitcase, the handle on your coffee mug, the handle on a water valve, etc.
