What is a coefficient in reaction?

The coefficients are the numbers written in front of the chemical formulas in a chemical equation, and tells how much of a certain substance is involved in the reaction. When there is no number, it is understood to be one. For example, the following equation, which represents the combustion of methane (CH4), could be read in a couple of different ways.

CH4 + 2O2 --> CO2 + 2H2O

One molecule of methane plus two molecules of oxygen produces one molecule of carbon dioxide plus two molecules of water.

or

One mole of methane plus two moles of oxygen produce one mole of methane plus two moles of water.