Positive transfer is the improvement or embellishment of current knowledge through the gain of additional information or education.
Typically this occurs when performance of a task improves as a result of performing a different but related task. It is essentially using the building blocks of previous to knowledge to learn more -- by learning something similar but different you can strengthen your previous skills as well.
For instance, this is what happens when a person continues to study and learn more about a subject. Once a person has a basic knowledge of a subject it is not difficult to add more knowledge of that subject. For instance, once a child starts to learn math every school year they learn different but similar concepts as time goes on. As they gain new knowledge and skills their original math skills typically become stronger than they were initially. Think about it, by the time people begin to learn algebra and geometry basic addition and subtraction seem a lot easier!