Positive space is the area of a painting or drawing that is the subject. Negative space is the complete opposite! It's all about the areas around, in-between, and behind the main subject such as the background. Negative space is important because it helps to balance out a painting or drawing. It also helps an artists to design a better composition and reveal the subject.
To get a better understanding... if your subject is small then your negative space will be greater and your positive space smaller. If your subject is large then your positive space will be greater and your negative space smaller.