One of the USCIS forms you apply for with your initial green card application is USCIS Form I-765
U.S. employers must check to make sure all employees, regardless of citizenship or national origin, are allowed to work in the United States.
If an individual is not a citizen or a lawful permanent resident, that individual may need to apply for an Employment Authorization Document (EAD) to prove he may work in the United States.
Certain classes of applicants for immigration benefits are entitled to an interim employment authorization document (EAD), which is a Department of Homeland Security (I-688B / I-766) document issued to certain categories of aliens who are not legal permanent residents (LPR).
An EAD gives a person legal status to work in the United States. It shows an expiration date (usually 2 years from date issued) and is renewable.