What is Mashout?
 
Before the sweet wort is drained from the mash and the grain is rinsed (sparged) of the residual sugars,
many brewers perform a mashout. Mashout is the term for raising the temperature of the mash to 170°F
prior to lautering. This step stops all of the enzyme action (preserving your fermentable sugar profile) and
makes the grainbed and wort more fluid. For most mashes with a ratio of 1.5-2 quarts of water per pound
of grain, the mashout is not needed. The grainbed will be loose enough to flow well. For a thicker mash, or
a mash composed of more than 25% of wheat or oats, a mashout may be needed to prevent a Set Mash/Stuck
Sparge. This is when the grain bed plugs up and no liquid will flow through it. A mashout helps prevent this
by making the sugars more fluid; like the difference between warm and cold honey. The mashout step can be
done using external heat or by adding hot water according to the multi-rest infusion calculations. 
A lot of homebrewers tend to skip the mashout step for most mashes with no consequences.