This is part of a broader topic of stoichiometry, of chemical equivalence. We know that #"Avogadro's number"# of #""^1H# ATOMS have a mass of #1*g# precisely.

And thus #"Avogadro's number,"# #N_A#, or #6.022xx10^23*mol^-1# is the link between the sub-micro world of atoms and molecules, to the macro world of grams, and litres, that which can measure on an analytical balance, or measure a gas or liquid by volume.

And so if I write the stoichiometric equation..........

#CH_4(g) + 2O_2 rarr CO_2(g) + 2H_2O(g) + Delta#

This tells me that one mole of methane, a mass of #16*g# reacts with a mass of #64*g# of dioxygen to give me a mass of #44*g# of carbon dioxide, and #36*g# with respect to water. Clearly, mass is conserved with respect to reactants and products, and the given chemical equation explicitly represents this........

To continue, sometimes (most of the time) we write that methane has a molecular mass of #16*g*mol^-1# i.e. #16*g# per #"Avogadro's number"# of methane molecules.

Can you tell me what #Delta# represents in the given equation?