Soft things are fluffy or yielding to the touch. A kitten's fur and your favorite fleece pajama pants are soft. So is spreadable cheese or the spongy ground of a wet field.
A soft voice is gentle, and a soft light or color is subdued or muted. A table with soft edges is rounded instead of sharp-cornered. Soft rain is mild. No matter how the adjective soft is used, it's always the opposite of words like hard, loud, bright or stubborn. It comes from a Germanic root that means "level, even, smooth, or gentle."