When you add "-ness" to an adjective, it becomes a noun. The suffix "-ness" means "state : condition : quality" and is used with an adjective to say something about the state, condition, or quality of being that adjective.
For example, redness is a red quality, and redness means "the quality of being red."
The redness in his eyes went away after he got some sleep.
Bitterness is a bitter condition, or "the condition of being bitter."
The breakup caused them to feel anger and bitterness.
Not all adjectives can be made into nouns using "-ness." Typically, if an adjective is in its -er or -est form, "-ness" cannot be added: higher and highest cannot become higherness or highestness. Typically, if an adjective is actually a participle of a verb, "-ness" cannot be added: washed and running cannot become washedness or runningness. Most other adjectives, however can be made into nouns by adding "ness."
Hope this helps 🍀