It's a bit of a roundabout way to say something, but it emphasizes that there is something, even if it's not much.
Here are some examples:
* "Il y a pas rien à manger." (There is something to eat.)
* "J'ai pas rien à faire." (I have something to do.)
* "Elle a pas rien fait pour réussir." (She did something to succeed.)
It's similar to the English phrase "not nothing", which has a similar effect.