A coefficient tells you the relative amount (in moles or in molecules) of that chemical used or produced in the reaction.
Here is an example:
In this equation, the "2" in front of
By the way, if there appears to be no coefficient, you assume that a "1" is there.
Then, on the right side of the arrow (the products) we see that one molecule of
This coefficients can have units that are either "molecules" or "moles" (if you are familiar with this unit). So we can read the equation as either
"One molecule of
"One mole of