Your job is to load as many of the required goods as possible onto the planes before time runs out. The more cargo holds you manage to fill, the bigger your reward will be. A cargo hold is fully loaded once you fill all its crates. You have 15 hours to load planes.
Send planes out once you think you've loaded enough goods. You can do that whenever you want, though you'll only be rewarded for cargo holds in which all crates are filled.
IMPORTANT NOTE: you have to send planes before time runs out to receive a reward. If you fail to do so and the plane takes off without your consent, you lose your reward even when all cargo holds are fully loaded.
Once planes take off, you have five hours to prepare for the next one: information about the next order will appear in the Airport window.