Form I-140, Petition for Alien Workers, is an essential step of your immigration process if you intent to come to the United States for work. The U.S. Citizenship and Immigration Services (USCIS) will handle this part of the process, and once approved will pass the case to the National Visa Center. Generally this type of application requires a skilled worker who is offered a job by an employer in the United States. More requirements and information can be found in the document Instructions for Petition for Alien Workers, published by USCIS.
What Form I-140 is and where you can lean more.