German adjectives provide more information about something or someone, an occurrence or a condition. They often describe nouns and personal pronouns. For many adjectives there is an opposite (antonym).

Examples:
Ich bin nicht klein. Ich bin groß.
Ich bin nicht dick. Ich bin schlank.
Ich bin nicht hässlich. Ich bin schön.
Bist du klug oder bist du dumm?

More:

When adjectives are placed after the noun or personal pronoun - like in the examples above - the basic form doesn't change.

If the adjectives precede the noun that they modify, then they are inflected. That means their endings change.