Law of Conservation of Mass
"The total mass of substances present before a chemical reaction is the same as the total mass of substances after a reaction."
So, matter is never created or destroyed.
Law of Constant Composition
"All samples of a compound have the same composition — the same proportions by mass of the constituent elements."
So, given a compound, there is a uniform ratio of masses within. For example, in water, there's always an 8 to 1 ratio of mass of oxygen to hydrogen.
Law of Multiple Proportions
"If two elements form more than a single compound, the masses of one element combined with a fixed mass of the second are in the ratio of small whole numbers."
So given a fixed mass, of say 1 g of carbon, and that's combined with 1.333 g of oxygen, and in another compound, 2.667 g of oxygen, the ration between them is 2 : 1. So, they can be expressed as two small whole numbers.