i ignored it.i think its for military service or something.....

Please read the quote from www.sss.gov "If you are a man ages 18 through 25 and living in the U.S., then you must register with Selective Service. It’s the law. You can register at any U.S. Post Office and do not need a social security number. When you do obtain a social security number, let Selective Service know. Provide a copy of your new social security number card; being sure to include your complete name, date of birth, Selective Service registration number, and current mailing address; and mail to the Selective Service System, P.O. Box 94636, Palatine, IL 60094-4636."

If you want to get US government job or apply for any Federal assistance in the future you need to sign up with the Selective Services before your 26th birthday. This information is collected by the US Government just in case they need to draft men for any war. However, there is no need to get scared as the US has not drafted anyone since the end of Vietnam war. So go ahead and register with SSS.

Selective Service does not collect any information which would indicate whether or not you are undocumented. You want to protect yourself for future U.S. citizenship and other government benefits and programs by registering with Selective Service. Do it today.

NON-CITIZENS
Some non-citizens are required to register. Others are not. Noncitizens who are not required to register with Selective Service include men who are in the U.S. on student or visitor visas, and men who are part of a diplomatic or trade mission and their families. Almost all other male noncitizens are required to register, including illegal aliens, legal permanent residents, and refugees. The general rule is that if a male noncitizen takes up residency in the U.S. before his 26th birthday, he must register with Selective Service. For a more detailed list of which non-citizens must register, see Who Must Register - Chart .

so hmm good to register.