Exactly what it says. I think if you have the motivation you wont have trouble saying what you have to...I would imagine stating that you understand flying for themis a lifestyle not just a job and THAT is why you want to fly for the Air Force not because your seeing it as a stepping stone to airliners. You can go on. Its almost like being in love, the words come from the heart

try to place yourself in their position.
But call them! ask them what needs to to be in the application form. Talk a bit with the people on the other side. If you can make them understand you have passion then they will make sure nothing goes wrong with the form. Thats how mine got accepted without me even giving a motivational letter.