The trash heaps are an area beyond Ember where the people of Ember put their trash and waste. Working there is an awful job. In the later years of Ember, the job of trash sifter was created to try and find useable items for the failing city. In the film people had to wear gas masks just to go near them.
Sometimes scavengers went there and stole junk that they could use in their own homes. The trash heaps were full of bugs and rats, and some people, too. Many people who wandered the Unknown Regions ended up at the trash heaps.