girl_with_a_bob

I went to the dentist for a routine cleaning/check up last week. At the end of my exam my dentist told me he'd like me to start wearing an essix retainer on my lower teeth. My question is what is an essix retainer, and why do I need to be wearing an essix(?) retainer? I am 25 years old, and have never worn braces or a retainer before, so I am unclear as to why I would need to wear one now. Any information is appreciated.