When flying Cargo and Scheduled flights – and Charter flights, though only when Worldwide Persistence is on/enabled – your departure airport must be the same as the arrival airport of your previous flight.
A Jumpseat enables you to change that.
With a Ticket, you can move from the airport where you currently are to any other one in the world. With Worldwide Persistence on, this also includes Charter flights.
Jumpseat tickets, though, are not free. They cost money. They're purchased using the virtual money you earn as a Pilot.
EXAMPLE: You're currently at KSEA - Seattle-Tacoma International Airport and wish to get to KJFK | New York John F. Kennedy International Airport. When you go to Flights > Jumpseats, you'll be presented with the screen below. Your Current Location will already be noted. Enter the airport where you want to go in the Transfer To box. In addition to a distance between the airports, a price will appear below it. If you want to purchase the seat, click the Purchase Ticket button. Repeat the process as many times as you wish before making a purchase decision.
If the price of the Ticket is more than the balance in your In Your Wallet account, you will not be able to purchase it.