Stealing from Pauln, AK Member who wrote the following which nicely allows one to envision the factors at play in getting a cartridge/stylus to track a record properly:
"Compliance is a measure of how hard the groove wall has to press on the stylus tip to get it to move.
Since the pressure and movement are changing through time, the usual attributes that come into play with accelerations apply.
In an automobile suspension, all the moving parts are classified as belonging to one of parts of the system - the sprung weight, and the un-sprung weight. In the case of a car, the un-sprung weight is the car itself, the suspension mounting, and the parts of the suspension that "don't move". The sprung weight included the wheel and hub, maybe part of the axle, etc. (the parts that do move).
It get a little tricky when considering the shocks, springs, and sometimes the axle. These things "move", but they are typically held fast at the un-sprung end. What happens is that the wheel itself is considered fully sprung, but the shock or spring is considered partially sprung (math is used to figure the equivalent sprung mass of the partially sprung components).
Anyway, on the turntable, the stylus, cantilever, and a portion of the suspension mounting, (and the coil if MC, or the magnet if MM, or the iron if MI) is the sprung weight; and the tonearm, balance weight, and cartridge mounting is the un-sprung weight.
(And if your turntable uses a suspension subsystem, there is a similar relationship between the tonearm/platter system (sprung) and the chassis (un-sprung).
Because of the geometry of the sprung weight (a tip at the end of a rod with the rod mounted at the other end with something connected to it - a coil, magnet, or iron), the usual way this is all described is effective tip mass. Which is to say, all the linkages and differential momentum and inertia and damping of the cantilever mounting is all rolled into one figure that describes how the groove wall would receive and respond to an equivalent isolated little mass in contact with the walls. The calculation reveals how much the groove wall "thinks" the tip weighs by how hard or easy it is to push and accelerate the tip.
But since the tip in the real world is connected to the rest of the system, the compliance needs to match the physical characteristics of the arm, the mounting, etc. because of the accelerations and inertia.
Sometimes it helps to visualize the extreme cases using a thought experiment.
Case 1 - Low compliance and light arm
Here the tip may be heavy and the cantilever very heavy and the mounting very tight and stiff. Let the arm be very light (like a soda straw). When the groove presses the tip, the tip resists deflecting in relation to the arm strongly and the whole arm moves. Now the whole geometry is wrong because instead of just wiggling the cantilever the whole arm is trying to wiggle and the effects of its geometry and mounting to the table come into effect.
Case 2 - High compliance and heavy arm
Let's say the arm is made of granite and weighs about ten pounds, but it is balanced just right and is floating on air bearings. What happens when the groove wall presses the tip sideways? The tip moves, but the arm does not. What happens when the groove presses the tip up? It moved up, but the arm does not. What happens after a few seconds of tracking? The tip gets bent to the side as the groove moves inward to the center of the record. The arm does not move. The tip finally mistracks and hops over the groove into the next groove and repeats this indefinitely. The compliance is too great for the deflected tip to move the arm. If the tip had a few minutes to apply its deflection continuously to the arm, you might see the arm begin to move, but that's too late, and it would take the same amount of time deflecting the other way just to get it to stop."
