Mob stacking compresses groups of identical mobs into one visible mob with a stack count. A stack of 30 zombies looks like one zombie until you start killing it, then the count ticks down and pays out drops and XP across those deaths. In a spawner room you are fighting the counter, not a crowd.
The gameplay shifts from entity management to throughput. Farms stay productive without turning the chunk into a lag pit, so you can run grinders, spawners, and animal pens without the server choking on AI and collisions. Building also changes: the bottleneck is how quickly you can kill and collect, not how many bodies you can cram into a space.
Details vary by server, and you feel it fast. Some setups share health across the stack, some only remove one mob per kill, and AoE like sweeping edge or splash damage can act differently than vanilla expectations. Wild mobs may or may not stack; servers that allow it everywhere can make fights feel like clearing a queue, while many keep stacking focused on farms to avoid annoying exploration.
Economies often lean on mob stacking because it makes output predictable. A stacked cow pen replaces the classic laggy mega-breeder, and spawner grinding becomes stable income instead of a performance gamble. Better-balanced servers keep caps, slow stacking near players, or tune XP so it stays a convenience feature instead of an early-game skip.
