A Green Card holder need to apply for a travel document, like a re-entry permit, if they want to travel out the U.S. for more than one year so they keep their residency status. If they don’t have it, being gone too long can make the government think they don’t want to live in America anymore, and it might mess up their residency.

You will need proof of permenent residency, valid passport, a reason to travel and sufficient funds for your trip. Your residency status will be temporarily suspended while you're outside the U.S. If you're returning within a certain timeframe (usually six months), your residency status is generally unaffected.