Soft light refers to light that tends to "wrap" around objects, casting shadows with soft edges. The softness of the light depends mostly on the following two factors:
Distance. The closer the light source, the softer it becomes.
Size of light source. The larger the source, the softer it becomes.
The softness of a light source can also be determined by the angle between the illuminated object and the 'length' of the light source (the longest dimension that is perpendicular to the object being lit). The larger this angle is, the softer the light source.