What's the relationship between supersaturation and temperature? In gravimetric analysis, what does supersaturation mean? And, how is it achieved by lowering the temperature, even though increasing the temperature makes a higher number of solute particles more soluble?
Supersaturation means simply that more of the substance is dissolved than should be. Supersaturation is an unstable condition. Given some "trigger" precipitation will occur and equilibrium conditions will then be restored. 

The trigger could be a seed crystal, scratching a glass rod on a side wall or a piece of dust which acts as a nucleation point.