My understanding of stoichiometric in this context is the proportion (ratio or percentage) of an element present - in this case how much NO is left after the air/fuel combustion process has taken place.
Air consists, mostly, of oxygen and nitrogen.
Fuel is a hydrocarbon.
In complete combustion of the appropriate quantities of fuel and air (in the stoichiometric ratio), all the carbon and hydrogen from the fuel combine with all the oxygen in the air to form CO2 and H2O. The nitrogen in the air goes in one end and out the other.
Unfortunately there are lots of reasons why this cannot be achieved in practice, with the result that the products of combustion include the above plus unburnt fuel, CO, and various oxides of nitrogen - NOx .
Yes, it's the optimum ratio. The key concept is definite proportions. Chemical reactants combine with each other in definite proportions. The composition ratio of a mixture of reactants is stoichiometric (for a given reaction) when the relative proportions of each match the coefficients in the reaction equation.
Stoichiometry is a term in beginning chemistry. In addition to all the other links provided so far, you can check a chemistry textbook or search the Web for <chemistry "stoichiometric ratio">.
When the mixture's composition ratio is not stoichiometric, there is a relative deficiency (excess) of some reactants. Therefore, a given volume of mixture won't react (be consumed) totally, there you won't get the maximum possible reaction outcome (whether that be heat or a product chemical), which is an inefficient state of affairs.