It's all about strength and sensitivity.
I'm not certain what the "I" stands for (probably "intrinsic") but the "M" stands for "Modulus".
modulus
Mod"u*lus, n.; pl. Moduli. [L., a small measure. See Module, n.] (Math., Mech., & Physics) A quantity or coefficient, or constant, which expresses the measure of some specified force, property, or quality, as of elasticity, strength, efficiency, etc.; a parameter.
In graphite fishing rods it is a measure of the strength and sensitivity. Sensitivity is measured by modulus in the industry. The more modulus a rod has, the more sensitive, light, and expensive the rod will be. IM6 graphite has about 35 million modulus, IM7 graphite has about 45 million modulus, while IM8 graphite has about 60 million modulus. Modulus is actually basically the tightness of the weave of the material that makes up the rod. Graphite having 30-60 million modulus is much more sensitive then the average fiberglass rod which has between 6 and 14 modulus.
Hope this helps.