It's a test that university graduates in the traditional engineering disciplines (civil, mechanical, etc.) usually take in the USA. After 5 years of work experience, you take another test, then your state's licensing board gives you a stamp and the credential "Professional Engineer" (P.E.). You can then take on liability for project documents that you stamp, so you charge clients/employers more money.
It's mainly useful if you work for a consulting company.
It's mainly useful if you work for a consulting company.