High-strength is a label that can be applied to a number of different 3D printing filaments for a number of reasons, so there is some confusion as to what it actually means!
There are 4 commonly used measures for strength in plastics:
There are some other measures of strength, such as the Rockwell Hardness Test or Elongation at Break however these are not as commonly used or listed.
The strength of a plastic will predominantly depend on the material it is made from and the strength of the molecular interactions between the individual chains of polymers. It will also be affected by whether a material is crystalline or amorphous. Strength can be enhanced through the use of additives like graphene or carbon reinforcement.