Performance is something that one can easily measure. How well does a person remember some facts? How well can an athlete perform a particular task? Learning is something that needs to be inferred from performance - it cannot be observed directly. There are many instances where learning occurs but performance in the short term doesn't improve, and there are instances where performance improves, but little learning seems to happen in the long term. Dr. Bjork tries to disentangle these two concepts and emphasizes that relying too heavily on short-term performance improvements leads to suboptimal learning.