Spark plug gap refers to the distance between the electrodes (center and ground) of the spark plug. The center electrode runs down through the middle of the spark plug and acts as a conductor for the high-voltage signal. The spark gap is necessary to create the spark - as the high voltage 'jumps' the gap - to ignite the air / fuel mixture inside the engine cylinder.
The spark plug gap has a specified distance; 0.035", 0.040", 0.050" and so on - to allow the optimum and strongest spark possible to insure the air / fuel mixture is properly burned within the cylinder. A weaker spark is created if the plug gap is too narrow, likewise if the plug gap is too wide - there may be a no spark or misifring condition.
For example, the spark plug gap specified for engine in your vehicle - ranges from; 0.028" to 0.044" - depending on the brand and style of the spark plug.