This means you must know the Laws of Evidence . Examples of physical evidence would include the gun used to commit the crime, other weapon present at the crime scene, money found, fingerprints, shoeprints, handwriting, etc. It may be the transcript of a telephone intercept. Authentication of the evidence is usually required along with expert testimony at times. The purpose of this stuff to to replace timely, expensive, and possibly prejudicial jury trips to the crime scene.
Any expert witness is by definition a forensic scientist, therefore.
A criminalist is a person usually with some expertise in a forensic science who goes beyond the evidence or applies their study of physical evidence to the reconstruction of the crime. Most criminalists are criminologists, but not all criminologists are criminalists. In some circles, the word criminalist is reserved for employees of crime labs. This test requires special pretrial hearings for scientific evidence and special procedures on discovery. Generally, there are stringent rules on validity and reliability error rates.