Airline Transport Pilot
This is the highest license you can get as a pilot and is required to act as the pilot in command (PIC) [captain] of a multi crew aircraft.
In the United States you have to be at least 23 years of age and have at least 1,500 hours of total flight time before obtaining an ATP.
It is common to fly as a commercial pilot (flight instructor, first officer, etc.) until you have the needed hours and then you do the written exam followed by a check ride. As airlines usually hire you to one day be a captain with the company some require you hold the ATP to get a job even though you fly as a first officer. This of course varies depending on the market situation and today all regional airlines will provide and pay for the ATP training.