If you are not a U.S. citizen, you may need permission to return to the United States after traveling abroad. This permission is granted through a travel document. Travel documents are also given to people who want to travel, but cannot get a passport from their country of nationality.
Refugee Travel Documents
A refugee travel document allows aliens who were admitted to the United States as refugees or who were granted asylum to return to the United States after travel abroad. The refugee or asylee should apply for a refugee travel document before leaving the United States. In some cases, immigration officials may issue travel documents to refugees or asylees who are physically outside of the United Stastes. See 8 CFR 223.2(b)(ii)
A refugee travel document does not guarantee admission to the United States. An alien with a refugee travel document is still subject to inspection and may be denied admission if the alien is inadmissible.