A stoichiometric AFR is the ratio at which, all of the oxygen in the intake charge is perfectly matched with the gasoline, leaving no unburned oxygen or gasoline in the exhaust. A lower than stoich results in unburned gasoline in the exhaust; a ratio higher than stoich results in unburned oxygen in the exhaust.
In theory this ratio is 14.7 for pure gasoline, but in practice the stoichiometric ratio may be slightly lower due to additives in the fuel.
See AFR for more information.
