It can refer to:
* The back of a person or animal: "J'ai mal au dos." (I have a backache.)
* The back of an object: "Le dos du canapé est en cuir." (The back of the sofa is leather.)
* The reverse side of something: "Tourne la feuille, regarde le dos." (Turn the page, look at the back.)
It's important to note that "le dos" is masculine, so you would use "le" before it.