After you have gained lawful permanent resident status, you may want to travel outside the U.S. and be permitted to return.  If this describes you, then Form I-131 is what you need to file to obtain your re-entry permit.  A re-entry permit means you do not need to obtain a returning resident visa from a U. S. Consulate or Embassy.

Once you obtain the re-entry permit, it is good for two years.

If you will be outside the United States for 1 year or more, you may want to file Form N-470, Application to Preserve Residence for Naturalization.