A hot liquor tank is, well, a tank in which you hold hot liquor! It's the vessel in which you keep your strike and/or sparge liquor. Often, it is directly heated somehow, either with flame or an electric element.
Remember, brewers don't use water in beer; as soon as water goes into the brewhouse with the intent that it ends up in the beer, it's called liquor. Water is for cleaning. It's not sparge water, it's sparge liquor. Thus, "hot liquor tank" or "HLT".