The suffix -ness is mainly added to adjectives and makes the adjective to a noun, meaning the quality or condition of what the adjective expresses.
"soft" has the noun softness, meaning the quality/condition of being soft.
As to the origin of - ness, I would like to add to what etymonline says (related with German -nis as in Zeugnis, certificate/testimonial) I hold the view that it is etymologically a left-right variant. If you read -ness backwards you get "sen", corresponding to German sein/das Sein, the being.
That some related words in one or two languages can be left-right variants is no official theory, it is rather a theory of mine. To back up this theory I have found about 250 incidents in a period of twenty years. The best known incident is Latin form-a and Greek morf-ae. Another example is English-German or German-German pot/der Pott and der Topf.