What is Fludrocortisone used for?

I was recently diagnosed with Addison's disease and type 1 diabetes after an adrenal crisis in Feb this year. I was immediately put on a dose of Hydrocortisone 20mg daily. I was recently up with my endocrinologist and he changed my dosage to 30mg daily and put me on a dose of 0.1mg of Fludrocortisone. I will not be with my endocrinologist again untill june and was just wondering why I am on fludrocortisone and what it is used for?