In this thermochemical reaction, what does the coefficient ½ represent?

A thermochemical reaction is a chemical reaction with an additional information of the heat of reaction. For example, this is the thermochemical reaction for the combustion of hydrogen gas:

H₂ (g) + 1/2 O₂ (g) → H₂O (l) ΔH = - 285.8 kJ

The coefficient 1/2 represents the number of moles of O₂ needed to combust 1 mole of H₂ gas in order to yield one mole of water.