From the equation of the chemical reaction, we find the ratio of the reaction.
Example:
N₂ + 3 H₂ ---> 2 NH₃
The underlined numbers represent the number of molecules that are needed of each compound in the reaction. From there you have the reaction ratio.
So in this example, there are:
1 molecule of N₂
3 molecule of H₂
2 molecule of NH₃
This means that it takes 1 molecule of N₂ reacting with 3 molecules of H₂ to form 2 molecules of NH₃.
Also it means that it takes 1 mole of N₂ reacting with 3 moles of H₂ to form 2 moles of NH₃.