A mash tun, also combined with a lauter tun, is a vessel in which you infuse your grains; usually a large cooler (or kettle) equipped with a false bottom and a valve with a spigot that allows you to draw off the sweet wort.
In order to extract the sugar from the grains, you mix them into the mash tun filled with water that is heated to 156 to 168 °F. Since the grains are at room temperature, this will bring this temperature down to around 148 to 158 °F, which is where the saccharification rest should occur. This is when the sugars are extracted from the grains.
Depending on the recipe, this rest will last about sixty to ninety minutes. At this point, it’s time to sparge into your brew kettle.