In the most simple terms, Daubert states that all expert testimony must be based on scientific foundation and not simply an opinion. While that may not seem important, consider this.
Experts purport to offer testimony that is scientific.
In order to qualify as 'scientific knowledge', an inference or assertion must be derived by the scientific method. Daubert at 2796. Scientific Methodology is Essential. This entails a preliminary assessment of whether the reasoning or methodology underlying the testimony is scientifically valid and of whether that reasoning or methodology properly can be applied to the facts in issue. In a case involving scientific evidence, evidentiary reliability will be based upon scientific validity.