Let us understand the Law of Conservation of Mass similarly. If you took the letters in the word, "eleven plus two" and re-arranged them, you would get the words, "twelve plus one".

Just the way letters are getting re-arranged here, atoms get re-arranged in a chemical reaction.

If you counted the number of e's in "eleven plus two", it would obviously equal the number of e's in "twelve plus one". Similarly, if you calculated the total mass of the substances before and after a chemical reaction, they would be the same because a reaction is just the re-arrangement of atoms.