I assume this question is about gravitational constant G used in Gravitational field.
The gravitational force of attraction between two masses m1 and m2 which are kept at distance d from each other is given by
where G is universal gravitational constant and its value is given by, G = 6.67408×10-11 m3 kg-1s2
when m1 , m2 are unit mass and distance d is unit distance, then we have F = G
Hence gravitational constant G is the gravitational force of attraction between unit masses which are kept at a unit distance from each other