Chemical equations must always balance due to the principles outlined in The Law Of Conservation of Matter. This scientific law states that matter cannot be created out of nothing nor can it be destroyed. Therefore, the same amount of matter, in the form of atoms, must exist both before and after the chemical reaction. The matter may change form or location, but it cannot disappear completely or appear out of nothing.
It is sometimes easier to imagine the atoms that make up a chemical equation as something larger. For example, if one is making salad, he might start with a head of lettuce, a whole tomato and a carrot. He needs to chop these vegetables up to make the salad, but he has not actually made some of the vegetables disappear just because he cut them into smaller pieces. New vegetables have not been created, the components of the salad have just been mixed together. The same is true of chemical equations which begin with a certain amount of one element and end with the same amount, on the atomic level, of another.
One of the simplest chemical equations is H2 + O2 -> H2O. That is, two atoms of hydrogen and one atom of oxygen combine to form a single molecule of water.