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.