(If the sentence read: "The wise king is kind to his subjects," "wise" would be a weak adjective).
This characteristic of Old English adjectives is important, because there are different declensions that are used depending on whether or an adjective is used in a grammatically strong or weak manner. A "declension" is simply a list of the different endings that go on a word to indicate that it is in a certain case (i.e., that it is fulfilling a certain grammatical function).