Originally Posted by cmbdiesel

The mash is all the grains steeping in the hot water, much like tea. When the tea, or wort, has reached the targeted goal (either starch conversion or specific gravity) then begins the runoff and sparge. Sparging is the introduction of hot water to the top of the mash as the wort is drained from the bottom. If you have fine ground grain or adjuncts like oats or rye in the mash, they can create a virtually impassable barrier to the wort as it tries to pass through the screen at the bottom of the mash tun. That is a stuck mash. Ideally, the screen is only there to keep the initial grains from passing through, and then the remaining grains create the actual filter. When the mash gets stuck, you really have no option but stirring or poking, either of which will introduce considerable cloudiness to the runoff.
Certainly not the end of the world, but can be concerning to some.