How does a balanced reaction show law of conservation ?

I will take an example just to make things a little bit easier:

E.g. 2H2 + O2 -> 2H2O

The reaction above is the reaction of hydrogen gas with oxygen gas which creates water.

As you can see, in a balanced reaction, the numbers of moles of each substance is the same before and after the reaction (for example, the hydrogen has the same number of moles before and after the reaction which is 4), therefore, if the numbers of moles are the same, so as the mass of the reactants and the products and this fact can give us an idea about conservation which states that the total mass is kept consistance and can't be destroyed or lost.

*hope it helps you