An adjective is a word which describes a noun. In English, for example, adjectives are words like ‘hairy’, ‘cute’, ‘annoying’, ‘futuristic’ and so on – there are thousands of them.
In German it’s a bit more of a faff to describe something because adjectives almost always need a different ending depending on which noun you are describing (that is, its gender and case). This is because in German the grammar gives us a lot of information about what is going on in the sentence – information which we get in English from context and implication – so even the adjectives have to work a little bit to help the sentence make sense.