Stack height is the distance from the pedal axle to the bottom of your foot — basically how much sole material sits between you and the pedal. A lower stack height means your foot sits closer to the pedal, which slightly improves power transfer and pedaling stability. That's why road racing and time trial shoes often have stack heights around 5-8mm.
For most road riders, stack height falls between 7-11mm, which is a solid balance of efficiency and comfort. Mountain bike shoes tend to run higher (10-15mm) because they need tread for walking off the bike. When comparing shoes, keep in mind that cleat type also affects your total stack — the same shoe can feel different with different cleats.
If you're a competitive racer chasing every watt, lower stack height is worth seeking out. For recreational riders, a 1-2mm difference won't noticeably affect your ride, so prioritize fit and comfort instead.