In an internal combustion engine, this refers to (here's a surprise) the mixture of fuel (Gasoline or Diesel Fuel, usually) and air that enters the combustion chamber and is detonated by the spark from the spark plug. (Or, as The_Custodian points out, in the case of a Diesel engine, the pressure itself generates the heat necessary to ignite the fuel.)

You want to have as much air and fuel as you can manage (if you're going for performance), but no extra of either. Unburned fuel is wasteful and can adversely affect the interior surfaces of the combustion chamber over time, and too much air ain't much better.

So you want to oxidize as much fuel as possible. This is what the carburetor (or in some engines, fuel injector) is for. They achieve the same end (the vaporization of fuel) in different ways, but do the same job - to effectively mix fuel and air prior to detonation.

So that's what is generally meant by the term "mixture" in this context.