Although he is not as famous as Toussaint Louverture, Dessalines was one of the main leaders of the rebellion that brought independence to Haiti. His major significance in the history of the world is that he was the leader of the rebellion (after Louverture was captured) when it succeeded in overthrowing the French and he was the first ruler of independent Haiti, eventually proclaiming himself emperor.
This leaves Dessalines with a legacy that has at least two parts. On the one hand, he is one of the main leaders of the Haitian Revolution. This revolution created the first black-led republic in the world. It was also the first instance of non-white people throwing off white oppression. Because of this, Dessalines’s legacy is partly one of resistance to oppression and, to some degree, racial equality and justice.
On the other hand, Dessalines also ended up proclaiming himself emperor rather than helping to create a republic. He ruled despotically. This made him unpopular enough that he was overthrown and killed. This means that he was not a second George Washington, helping to create a stable and long-lasting (and relatively just) government.
Dessalines is significant in world history because he helped to lead the Haitian Revolution and was the first ruler of independent Haiti.