If a reaction taking place in aqueous solution produces an insoluble product, that product will precipitate, forming a solid product at the bottom of the reaction vessel. Precipitation often happens with double replacement reactions, and one of the most dramatic of such reactions is the reaction between lead nitrate and potassium iodide:
Both of the reactants form colorless solutions, but if these solutions are poured together the lead iodide product is a dramatic yellow solid which precipitates out of the solution. Both of the reactants are ionic compounds and if it were not for the precipitation, the reaction would not proceed - you would just get the four ions in equilibrium.
From experience, empirical rules have been developed to help anticipate when a precipitation reaction will occur.
Chemical reaction concepts