The Engine Computer detected a problem that could adversely affect tail pipe emissions and set a diagnostic fault code in memory. Your mechanic will connect a scanner, (hand-held computer) to read those codes which will point to the circuit or system with the problem, not necessarily the defective part. It's the starting point in the diagnostic procedure. He will turn the light off after the repairs are completed.
Depending on the severity of the problem, the light could go off by itself once the problem is corrected but the code will remain in memory for a while.
Be sure your gas cap is tight. The fuel supply system goes through a self test to look for leaks and a loose gas cap is a common cause of leaks.