A brine tank grid is simply a piece of plastic with many holes drilled in it. This piece of plastic sits on small legs in the bottom of your brine tank.  The original purpose of the brine tank grid was to allow for the use of a smaller brine tank by displacing salt and allowing the brine tank to hold more water. 

Most people find grids useful to help keep salt up and out of some of the water remaining in the brine tank after regeneration (especially with “post-fill” valves like many of our Fleck models). Keeping salt out of the water helps prevent mushing.  You can also avoid mushing and bridging by using a high quality water conditioning salt (read about that here).