The word prehistoric is an adjective.
It is used to describe something that relates to the period of history before written records were kept. For instance, when we talk about prehistoric times, we are referring to an era that encompasses early human history, including the Stone Age and other ancient periods where societies existed without written communication.
Adjectives are words that modify nouns, providing additional information about them. In the case of prehistoric, it helps specify the time period we are discussing, and thus, it helps to create a clearer understanding of the subject matter.