AFR is the mass ratio of air to fuel present in a cylinder prior to combustion. For gasoline, "stoichiometric" is 14.7 to 1 (aka lambda 1). Lower ratios are "rich" (too much fuel), higher ratios are "lean" (too little fuel).
When tuning forced induction cars to run on pump gas, rich fuel ratios can be used to cool the intake charge to assist in reducing knock. It is an effective (if wasteful) way of gaining a bit of timing when race fuel is not available.
Rich mixtures are also commonly used to reduce EGTs to prevent turbo damage.