Sugar dissolves by water molecules surrounding sugar molecules and separting in from the rest of the solid. When sugar is added to water, energy is used up to break apart the molecules in the sugar. The water molecules then begin to spread out to make room for the sugar. This way the sugar gets dissolved in the water.
When the water is hot, the sugar molecules break apart faster, and the water molecules move around the sugar faster. With increased temperature, the water molecules are moving more rapidly, increasing the speed at which hydrated sugar molecules are transported away from the solid, increasing the rate at which the solid dissolves. In cold water, this process is slow, hence the sugar does not get dissolved faster.