When you temper a blade you use a set temperature or temperatures for a set period of time. This step is reapeated two or three times. The goal of tempering is to reach a set rockwell hardness that matches the blade design and its intended use.
Overtempering would be using to high of a temperature, ending up with a blade that is softer than the knifesmith wanted.
You can temper a blade at 400 degrees for one hour or 100 the blade will still be the same hardness.