With a paid seat reservation, you can select your own seat(s) well before departure. This way, you can make sure you sit next to each other or to have a seat with extra legroom. The earlier you reserve your seat(s), the bigger the chance your desired seat(s) are still available.
If you are selecting your seats when checking in online, you can only select the available seats that are still left. You will have less options, but sometimes these seats are free. You can often check in online 24 hours before departure.
If you are checking in at the airport, the ground stewardess will select your seat(s). The choice is limited.