modulus short for an enginnering term 'modulus of elasticity' This is one of the basic mechanical properties of a material. It's a measure of how much a material changes shape when subjected to a load. In engineering terms the modulus of elasticity = stress/strain. The higher the value the 'stiffer' the material is.

For any given material the taper design, wall thickness and fiber orientaion will predict the action of the rod, much more than the material it's made from. I could build very stiff rod from a low modulus material such as wood (like a broom stick),or a very soft rod from high modulus material such as steel (like a radio antenna).

With that said, for the same flex charactoristics a rod made from high modulus material would have less mass than the same rod made from lower modulus material. This is because it would require less material to achive the same flex charactoristics. I think this is why rod manufactors are always advertising the 'high modulus' graphite.

It means they can build a stiff fast rod with a thinner, lighter (and more brittle) wall section. The new high modulus graphite rods weigh very little compared to older fiberglass or cane rods. They also break more easily since they have much thinner wall sections.

Just because a rod is light, fast or made from the latest most expensive material doesn't mean it feels good in the hand. I suggest you actually cast a rod before you make your choice. There are a lot of lower modulus rods that cast beautifully. And some of those fancy high modulus rods feel like a club in my hands...

Stiffer doesn't always mean better, but in material properties terms it usually means more expensive.