In a chemical equation, the big numbers in front of the chemical formulas are called coefficients. They represent the relative number of molecules or moles of each substance involved in the reaction.

For example, consider the following chemical equation:

```

2H2 + O2 -> 2H2O

```

In this equation, the coefficient 2 in front of H2 indicates that two molecules of hydrogen gas are required for the reaction. The coefficient 1 in front of O2 indicates that one molecule of oxygen gas is required. And the coefficient 2 in front of H2O indicates that two molecules of water are produced.

Coefficients are important because they tell us the stoichiometry of a reaction, which is the quantitative relationship between the reactants and products. Stoichiometry is important for understanding the efficiency and yield of a reaction, and for designing experiments and industrial processes.