An anchor is the point of a draggable which has to be on the droppable for the draggable to be considered "inside" that droppable. Think of it as the tip of the pointer of your mouse. The mouse pointer is a shape, and its anchor point is the tip.
Drag the draggables over the droppable and see when the droppable reacts.
The Quick Brown Fox jumps over the lazy dog
The Quick Brown Fox jumps over the lazy dog