In فَبِكَلِمَةٍ طَيِّبَةٍ, كَلِمَةٍ is the noun and طَيِّبَةٍ is its adjective. Both are feminine and in the genitive case. A kind word here means more than empty praise: truthful encouragement, a respectful response, or useful reassurance at the right moment.
Example 1: Listen attentively to a distressed person and offer encouragement.
Example 2: If unable to provide money, do not promise what cannot be delivered; respond honestly and gently.
Kindness in speech costs no money, but it still requires sincerity and responsibility.