3 and 5 do not make sense, but I can answer the rest.
1. Reactants are on the left side, and the products are on the right side. Reactants, hence the name, react to form products. So the Cd(NO3)2 compound and the H2S compound are reactants, and the ones on the other side are products.
2. The little letters in parentheses tell you the state of matter. (s) is solid, (aq) is aqueous, (l) is liquid, and (g) is gas.
4. The law of conservation of mass is important, because in reactions mass never disappears just like energy. Mass is never created or destroyed. Mass does not randomly appear, and if the products have less mass than the reactants, it is not that the mass was destroyed.