This is a question that comes up when people first start learning Creole. I will give the short answer in this article but you can learn much more by diving into the information on Wikipedia.
Think of a “creole” language as a “re-mix” of the original language. After hundreds of years these become native tongues in their own right and well suited for their own special cultures.
Haitian Creole is the language of Haiti. When the French enslaved millions of Africans over several hundred years, they assimilated a people from many different languages into one small colony. The colonial overseers only taught the most essential parts of their French language to these forced workers. This basic French combined with African, Spanish, and eventually English influences to form modern Haitian Creole.
The francophone language still holds strong influence, as French is the second official language of Haiti. Creole is a fully formed language with over 12 million native speakers, it has roots in French but has advanced to fit the unique culture of Haiti.
Please never call Haitian Creole a “dumbed down” French. That is a historically ignorant statement loaded white supremacy and disrespect of the people of Haiti.
French Creole is a category of languages, see this Wikipedia article for a list. These are also the result of French colonization around the world. However, few of these have achieved the extensive development and widespread usage enjoyed by Haitian Creole.