Because German nouns are gendered, adjectives have endings that correspond to the gender of the nouns they modify. The endings are either weak or strong, depending on what precedes the adjectives. Definite articles (der, die, das) are strong, meaning they indicate gender; indefinite articles (ein, eine, einen) are weak, which means that gender is at times ambiguous. For this reason, adjectives following weak-ended, indefinite articles must have a strong ending, and, vice-versa, adjectives following strong-ended, definite articles have weak endings.
In addition to adjective endings, there are comparative and superlative forms of adjectives used when comparing two or more things.