Green Card is official document issued by USCIS that identify the holder as a lawful permanent resident of the United States. The green card could be used for work authorization.
Usually, there is holder's signature in the left bottom corner of a green card. However,
children under the age of consent or individuals who are physically unable to provide a signature are not required to provide their signature.
Since February 2015, the signature requirement for people entering the United States for the first time as lawful permanent residents after obtaining an immigrant visa abroad from a U.S. Embassy or consulate is waived. When a Green Card has been issued without a signature, the card will say “Signature Waived” on the front and back of the card where a signature would normally be located.