In a normal petrol engine, the pistons inside the engine move up and down vertically, or on a slight angle in the case of a “V” engine.
In a boxer engine the pistons lie flat and move in and out horizontally instead.
The primary advantage of a boxer engine is that the weight of the engine is lower down than in a normal engine.
Having the weight of the engine lower down in the car contributes to a lower centre-of-gravity for the entire car, which improves the handling of the car.
Porsche and Subaru are two prominent car brands which use boxer engines in their cars today (the Toyota 86 is also fitted with a Subaru boxer engine).