When a wine is described as "hot," that means the alcohol level is unbalanced.  If you were to sniff a "hot" wine, most likely the first thing your nose would smell would be alcohol.  Tasting a "hot" wine would lend the same results. 

Why is this so and why is it called "hot?"

If you've ever drunk hard liquor, you might have felt what most people feel: a burning sensation in their throat (I.e. straight-up vodka).  When a wine smells of alcohol, it gives many people the same mouth feel, even if it doesn't produce the same warmth down the gullet.

Wine becomes "hot" for many reasons.  Sometimes the wine needs to be decanted and sometimes the wine has been decanted too long.  A lot of times people drink the wine at room temperature or warmer, making it appear "hot."  The winery may have produced a bad bottle or case or even the whole stock is just unbalanced because of poor vinification (the process of making wine).