Question

What do the coefficients located before certain molecules in each chemical equation represent?

Answer 1

it represents the number of moles required of that molecule to balance the chemical equation, which means to have the reaction chemically happen and goes to completion.

For example:
CH4 + O2 --> H2O + CO2 that is not balanced

with the coefficients located
CH4 + 2O2 --> 2H2O + CO2 now with the coefficients the number of oxygen and hydrogen on each side are equal