How often do you see rod manufacturers claim carbon fibres of “High modulus”? When a scientist or engineer wishes to describe the strength of a material the word “strong” can mean several things:
(a) how much force is needed to break it- ultimate strength
(b) how well can it resist sudden shock forces – toughness
(c) how little will it deform when a force is applied
It is this last one that is measured by the “Modulus of elasticity” or modulus. Most materials that have a high modulus are also brittle e.g. glass and ceramics. Carbon fibres have a very high modulus but are also brittle in themselves but by embedding them in a tough material such as epoxy resin or nylon the tackle makers produce the compromise of stiffness and toughness we require.
Simply put, the higher the modulus the stiffer the material will be but it may be less resistant to abuse.