The Re-entry Permit is a travel document similar to a certificate of identity issued by U.S. Immigration Department to U.S. lawful permanent residents to allow them to travel abroad and return to the U.S. It is a passport-like booklet with a blue-green cover with the words "TRAVEL DOCUMENT" on the front. The main purpose of the re-entry permit is to allow permanent residents to leave the U.S. for an extended amount of time without abandoning their permanent residence.
If you are a Bangladeshi-U.S. lawful permanent resident (green card holder), and you need to spend a year or more outside the United States, you would be wise to get a travel document called a "Re-entry Permit" for your return.
Although you are certainly allowed to travel, and a green card (plus your own passport) is normally all you need for entry upon returning (and you'll still need to carry those), the situation changes a bit if you spend a long time outside the United States. The immigration authorities will wonder whether you meant to abandon your U.S. residence. In fact, after one year, your green card alone becomes invalid for U.S. entry. A reentry permit helps prove that you did not mean to give up your green card.
A reentry permit is also useful for another purpose: when you cannot, or do not wish to, get a passport from your home country. The permit (which looks very much like a passport) can be used in its place.
A reentry permit is typically valid for two years from its date of issue. There's an exception for conditional residents whose residency expiration date will come before that amount of time is up. They will be given a reentry permit good for only as long as their conditional residence lasts, and will be expected to apply for and obtain U.S permanent residence before requesting any more reentry permits.
You cannot extend the time on your reentry permit. If it expires, but you haven't yet finished with whatever took you outside the U.S., you will need to return to the U.S. and apply for a new one.
Please keep in mind, you must apply for your reentry permit before leaving the United States.