Help me understand compliance!

Buy a HI-Fi NEWS TEST LP and measure your actual tonearm cartridge combo, you will see where is the resonance frequency really is. 

" Tracks 2 & 3: Cartridge & Arm, Lateral & Vertical Resonance Test
These two tracks are used to test the resonant frequency or your tonearm and cartridge combination in both the vertical and horizontal domains. These tracks offer both a visual and auditory indication of the resonant frequency; the stylus will “wobble” and the test tone will warble. A resonant frequency between 8 - 15Hz is ideal.'" 

Rega table.

Get a Rega cartridge that's in your budget-simple. Enjoy records.
The more you find out, the more unnecessary audio neurosis you'll develop.

The matching of cartridge compliance to tonearm effective mass is quite "plastic", in that there is a lot of room to make them match, if you use the standard goal of achieving a resonant frequency of 8 to 12Hz.  Plus, in the modern era, we are inundated with "medium mass" tonearms and cartridges with manageable compliance numbers, so for the most part if you are using new OEM stuff, you needn't worry too much.  MC, who discounts the importance, actually uses a tonearm that matches well with his cartridge, which is maybe why he can say he pays no attention to the issue.  If you look at the equation for the resonant frequency (Fr), it depends inversely upon the square-root of the product of tonearm mass, M, multiplied by the cartridge compliance, C, at 10Hz, as Chak says.  If you go on line and look up one of the calculators for Fr, you can fool around with the values for M and C and quickly see there is a lot of room for varying one vs the other parameter and still being in the OK range. This is because you are taking the square-root of the product of those two parameters, which mathematically jams the result into a narrower band of values that tend to produce an acceptable outcome.

By the way, "weight" as we commonly use the word, has little to do with the calculation.  "Effective mass" is a product of the distribution of the mass of the tonearm, starting from the counter-weight to the pivot, and from the pivot to the headshell and cartridge, to include the mass of the cartridge and the mounting hardware.  Don't knock yourself out with this stuff.  Probably your Rega tonearm is in the broad category of "medium mass".  Then it depends upon how much your cartridge adds to M and what is its C.

compliance relates to the stiffness of the cantilever and suspension.  
a higher compliance is more flexible.  lower compliance is stiffer. 
if you have a heavy truck you would want a stiffer suspension for more stability.  if the suspension was too flexible the truck would lean and sway on the road.  
an overly stiff suspension on a light car would also be unstable because the tires could lose contact with the road if it was bumpy. 
a low mass tonearm would be a bad match with a stiff, low compliance cartridge because trackability could suffer.  
a high mass tonearm would be a bad match with a flexible high compliance cartridge because the position of the stylus would be fighting against the higher inertia of the heavier tonearm, e.g. the suspension could be over stressed trying to track a warped record.  
soundsmith has a very good chart that matches cartridges of diffrrent compliances to a range of tonearm masses.  

Words of wisdom, short and sweet:

Rega table.

Get a Rega cartridge that's in your budget-simple. Enjoy records.
The more you find out, the more unnecessary audio neurosis you'll develop.