In commercial aviation, most airlines and many freight companies will have two persons in the cockpit of flights, due either to company policy or mandates of aviation authorities. While the pilot or pilot-in-command is responsible for the flight, the "first officer" is his/her second-in-command who assists the pilot in in various capacities throughout the flight.
Prior to takeoff, the first officer assists in conducting all safety checks and instrument checks. If manual logs are kept, the first officer is generally tasked with noting them, and he/she also assists the flight crew in helping passengers aboard and keeping the flight on-schedule.
In the air, the first officer must be available to relieve the pilot at any time. He/she must be fully capable of piloting the craft when called upon to do so, and should be familiar with all operations and communications as necessary. The first officer typically assists the pilot and acknowledges instrument readouts and navigational information as required, but must stand ready to handle the craft if the pilot needs a break or is somehow rendered unable to continue flying.
Those in this position also assist the pilot in all landings and communication with controllers at destination airports. Of course, they must be comfortable with landing and taxiing the aircraft, and may be called upon to assist customers with debarking the plane to help other flight crew to keep the plane's schedule on-point.
To work as a first officer, a person must be fully qualified by regulatory aviation agencies as a pilot for the particular aircraft he/she is expected to fly; this includes licensing, drug tests, and logged flight hours that may be in the thousands. Many first officers receive flight training and experience in the armed forces, where the experience needed for commercial aviation is most readily acquired. While officers assigned to short routes can expect to be home often, they should generally expect a great deal of overnight travel.