This is one of the shortest USCIS forms, it consists of only 5 pages. You can use Form I-131, Application for Travel Document, to apply for a re-entry permit, refugee travel document or advance parole travel document.
You should submit Form I-131 to USCIS and get it approved before traveling abroad, including trips to Mexico and Canada, thus you will be able to re-enter the United States. For example, this form is required in case you plan to leave the USA while your Form I-485, Application to Register Permanent Residence or Adjust Status, is still pending.
A re-entry permit allows a lawful permanent resident or a conditional permanent resident to apply for admission to the USA upon returning from abroad during the permit’s validity, without obtaining a returning resident visa.
A refugee travel document is issued to an individual who has valid refugee or asylee status and who wants to travel outside the USA. The document is required to return to the USA.
Advance parole document is intended for some other immigrant categories, such as adjustment of status applicants (according to Form I-485), individuals with temporary protected status, U visas and individuals with a grant of LIFE Act Family Unity Provisions benefits.
Meanwhile, please do not confuse Form I-131, Application for Travel Document, with Form I-131A, Application for Travel Document (Carrier Documentation). Form I-131A is intended for lawful permanent residents to apply for a travel document if they are returning from temporary overseas travel and their green card or reentry permit was lost, stolen or destroyed.