It’s a service that allows passengers who have purchased their tickets from the relevant airline to complete their transactions online more quickly and easily before they go to the airport. When you check in online, you don't have to wait in line for your check-in process at the airport.
You can also select your seat number by following the online check-in steps on the relevant airline page.
While many airlines do not charge a fee for online check-in, some airlines may charge for specially preferred seats.