Shock pack is the rapid rise in hydraulic resistance inside a shock absorber when the piston moves at high shaft speed, such as hitting a sharp jump face or flat-landing.
Low-speed chassis roll in a smooth sweeper pushes oil gently through the shock piston holes, whereas slapping a jump face forces oil through those same holes several times faster. Smaller piston hole diameters (for example, switching from 2 x 1.7 mm to 3 x 1.4 mm with similar total hole area) increase turbulence and create more pack: the car resists bottoming on big landings while still rolling plushly in slow turns. Too much pack makes the tires skip and chatter across square-edged chop on blown-out dirt; too little pack lets the chassis slap the ground and bounce out of rhythm sections.
Understanding pack lets you fix bottoming on big jumps with piston hole size instead of over-thickening shock oil and ruining slow-corner traction.