A grist hydrator is a useful piece of beer brewing equipment that wets your crushed grain as it drops into the mash tun. Grist hydrators are used to minimize doughballs in your mash tun.  This is important because doughballs keep the hot liquor from mingling with a portion of your crushed grain.  If the liquor is not able to access all of the grain, it will not be able to effectively activate your enzymes for conversion. 

Tip:  Using a mash paddle can also help to get rid of any doughballs in your grain.