Step 1 - Understanding the law of mass conservation
A chemical reaction may be defined as the rearrangement of atoms to form new molecules. Let's pay close attention to this word: rearrangement. It means that all the atoms contained in one side of the equation must appear at the other side. No atom can be created and no atom can just disappear.
If we have, let's say, O, N and H in one side of the equation, only O, N and H can appear at the other side as well. That's why we define a chemical reaction as a rearrangement. It's like building two different things from the same lego pieces.
This is known in chemistry as the law of mass conservation. Since no particle can disappear (or be created), the total mass, i.e., the sum of the masses of all particles, is equal during a chemical reaction, given that the system is closed (no gas can escape).
Step 2 - Understanding the given system
Since, as we saw, the same atoms (elements) must be present at both sides, no new elements were introduced (nor they could have been).
The gas that was formed, CO2, contains C as well as two O atoms. It must have come from the reactant that has a C atom bonded with two or more O atoms. It came thus from NaHCO3.
Since the system is closed, the gas won't be able to escape. Therefore, we expect no change in the mass of the system. If we could weight this reaction by putting the vessel over a scale, the weight wouldn't change during the whole reaction. The mass is conserved.