The law of conservation of mass states that whatever goes in comes out the same. That's not the exact concept, but it'll help you remember it.
Here's another way of thinking about it.
Okay, so, we have:
Reactants ⇒ Products
The reactants form into products, but stay the same.
What does this mean?
This means that the mass is conserved. When mass is conserved, the energy is also conserved.
In other words, nothing has changed.
Now, lets relate that to balancing equations.
When you try to balance an equation, what is your goal? ⇒ To get the reactants to equal the products..? Right?
Ah, yes!
Now we have the exact meaning→→→→
The law of conservation of mass states that mass is always conserved in a chemical reaction.
Hope I helped! Please feel free to comment with questions! :)