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One of the fundamental principles of chemistry is the conservation of matter. In fact, this is so important that scientists call it the law of conservation of matter. The law states that the total mass of the reactants is equal to the total mass of the products. In other words, you cannot create or destroy mass during a chemical reaction. 
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The term conservation means unchanged. When you hear that "matter is conserved during a chemical reaction", it really means the total number and type of atoms has not changed before and after the reaction. Every single reactant atom must be accounted for in the products. Reactant atoms do not disappear; instead they are rearranged to make products.
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For example, consider the photosynthesis reaction which takes gaseous carbon dioxide from the atmosphere, liquid water from the ground and produces solid glucose and gaseous oxygen. This reaction uses the energy of the sun to reorganize six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms from one configuration in the reactants to another configuration in the products. The same atoms appear on both sides of the reaction and that is why mass is conserved.
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Conservation of matter seems obvious today but it was not obvious at all when chemistry was being first understood. The problem is that scales do not weigh gases. The masses of each compound in the reaction are shown above. If both the CO2 and O2 were undetected it would look like 108.1 grams of water became 270.1 grams of glucose!
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