This topic is one in which the fundamental differences between SI and CI engines manifest themselves.
All SI engines have air/fuel mixtures existing in the cylinder before combustion is initiated by the spark plug. These mixtures may exist continuously in the inlet manifold (in the case of carburettor or 'throttle body injection' systems). Or, they may exist only after each cylinder begins its induction stroke (in the case of 'traditional' individual inlet tract injection). Or they may exist for only a short period in each cylinder before the spark plug fires (in the case of Direct Injection).
If such an air/fuel mixture exists, it follows that the quantities of air and fuel must exist in proportions which can be expressed as a ratio - the air/fuel ratio. The stoichiometric ratio for air and petrol (gasoline) is roughly 15:1. That is, for complete combustion without excess air, we need 15 times the quantity of air as the quantity of fuel. If there is less air in relation to fuel (say 13:1 ratio), it is called a 'rich' mixture. If there is more air in relation to fuel (say 17:1 ratio), it is called a 'lean' mixture. SI engines will only operate over a fairly narrow range of air/fuel ratios - if it is hugely rich or lean, it won't ignite at all. Therefore, even at idle, a close to stoichiometric ratio must be maintained and, to keep the engine speed at 'idle' rpm, the mass of both air and fuel ingested is severely restricted by the throttle. In fact, the main throttle valve is just about fully closed and a relatively tiny bypass passage allows a small quantity of air (or air/fuel mixture, the case of carburetted engines) into the engine.
In SI engines, close to stoichiometric combustion is desirable under steady cruising conditions, to give optimum economy, and most engine management systems constantly sample the exhaust gas stream to keep the quantity of excess air (oxygen) at a very low but measurable level. However, during acceleration, better performance is obtained with slightly rich mixtures and the EMS will 'enrich' the mixture under these conditions. What must be avoided at all costs, however, is excessively lean mixtures, especially under heavy load conditions. The lean mixtures cause far higher peak combustion temperatures and lead to rapid erosion of the exhaust valve and valve seat
and, in extreme conditions, can cause a hole to be burnt in the piston crown.