Single Malt Scotch is a type of single malt whisky, distilled by a single distillery in a pot still, using malted barley as the only grain ingredient, in Scotland. As with any Scotch whisky, a Single Malt Scotch must be distilled in Scotland and matured in oak casks in Scotland for at least three years and one day (most single malts are matured longer).
"Single" indicates that all the malts in the bottle come from a single distillery. Multi-distillery malts are usually called "blended malt", "vatted malt" or "pure malt".
"Malt" indicates that the whisky is distilled from a single "malted" grain. Not all grains can be malted (rye is another grain which can be malted) but in the case of single malt Scotch, barley is always the grain used.