In this instance the negative space (the space AROUND the objects) has been filled in while the positive shapes (the object you are looking at) has been left blank.
Negative space is important in drawing. If you accurately capture the NEGATIVE spaces you will also get the form of the object you are looking at correct. Look at how much information can be provided about space and detail by just observing the negative space in this drawing.
The overall composition of your drawing will improve if you consider the placement of your negative spaces. Notice that each corner of this composition is different.
Properly capturing negative space can help define detailed areas of your drawing.
In class today you will create negative space drawings by observing a still life in front of you and filling in the negative space only.