Foreign nationals who are granted lawful status in America, may travel abroad with their Green Cards for a limited period of time but if they remain abroad for an extended time period, they may not be able to get into the country. Hence, Green Card holders who wish to remain in a foreign country for a prolonged period of time, may obtain re-entry permits and remain abroad for less than two years, by filing Form I-131. If you hold a valid Green Card and if you plan to travel afield for not more than a year, you need not obtain a re-entry permit but if you feel that you will not get back to America within a year, you will have to get a re-entry permit before leaving the country. Remember that your permanent resident card will become invalid if you live abroad for more than 365 days without a re-entry permit. Similarly, if you live in a foreign country and plan to become a resident of that country and give up your residence in America, the US immigration officers will consider that you had abandoned your lawful status in the country and then your Green Card will be revoked.
This re-entry permit will help you to prove that you do not have plans to relinquish your lawful status. Similarly, after obtaining this permit, you can remain abroad for more than 365 days but not more than 2 years. You need not get a returning resident visa to get back to America, if you hold a re-entry permit that is valid for two years. Even if you do not hold a passport, you can travel outside America with this re-entry permit and this permit is similar to a passport and it has exit and entry stamps.
In order to get a re-entry permit, you will have to file the USCIS Form I-131, Application for Travel Document, ahead of your travel and remember that you cannot apply for and obtain this permit while in a foreign country. If you remain in a foreign country for a year and more without a re-entry permit, you may be questioned and you may be required to appear before an immigration judge.
You may travel abroad after applying for a re-entry permit and after you appear for biometrics collection. And in such cases, you may request the USCIS to mail your permit to an overseas consulate from where you can collect it. But if you travel before appearing for biometrics collection, your application will be denied. Hence it is recommended to file Form I-131, in advance. Remember that you must apply for this permit only if you plan to travel for more than 365 days and you can use your Green Card, if you are planning to travel only for a short period of time. Moreover, USCIS does not extend existing re-entry permits and these documents are valid only for a two year period. If your existing permit expires, you may apply for a new one but the USCIS will not issue a new permit if your existing permit is still valid. However, you may file a petition for a re-entry permit, if you had misplaced your existing permit.