High-speed damping, or pack, describes how shock damping increases in a non-linear fashion as the speed of the piston increases.
Now, let's try that in English. To understand pack, let's first look at low-speed damping. When your car is moving across smaller bumps or rolling in a turn, your shock pistons are moving at a relatively low speed through the oil in the shocks, so this is referred to as low-speed damping. At low speeds, the amount of damping force that the piston encounters as it moves through the shock fluid is roughly proportional to its speed, that is, if the piston is moving a little faster, the damping forces are a little stronger, working a little harder to slow the piston's movement (slowing the roll of the car, or moderating its movement over bumps.) In the fluid dynamics world, this is known as "laminar flow."
However, things get interesting when the pistons are forced through the shock oil at very high speeds - it results in high-speed damping (due to non-laminar, or turbulent flow) which is actually much stronger than low-speed damping. That is, high-speed piston movements are met with dramatically greater resistance than low-speed movements, generating what is referred to as "pack" describing how the oil "packs" up and strongly resists the movement of the piston and further compression of the shocks.
So, how is this all helpful, you ask? Well, you can use pack to improve your car's handling. Pack is particularly useful for off-road cars - pack can help keep your car from bottoming out when taking off from or landing after large jumps. It's also possible to use pack on smooth tracks to make the car feel more stable and easier to drive.
To create more pack in your shocks, use smaller holes in your pistons.