The simple part of the ollie is that when you are moving in a certain direction your board and your body will be more stable and easier to control. You will stay in line and be less likely to move out to the sides if you are rolling.
I look at what I think the science of the ollie is on this page. This is more of a story of what I think is happening. It hasn't been tested in a lab my people who really know science. Maybe one day I will get that chance.
I call the ollie movement of the board an open lever.
A lever is a rigid object that is used with a pivot point to multiply the force that can be applied to another object.
Using the back wheels as the pivot point you push down the tail moving the front of the skateboard into the air quickly.
The skateboard keeps going up into the air and then you open or break the lever by hitting (or popping) the tail on the ground.
This changes the angle that the skateboard is travelling up at. It causes the skateboard to leave the ground.
The front of the skateboard is still travelling up and the back has left the ground.
Using your feet you catch the board in the air. It is still moving up and appears to stick to your feet.
With the help of the grip tape on the top of the board you can level it out and hold it while it starts to go back down to the ground.
This lever motion happens quickly and it appears that the board does not actually leave your feet. When in actuality it does leave your feet for a fraction of a second.
This article on the physics of the ollie covers things in a different way than I do. With some good images to explain. I pulled one to show what they are getting at.
If you are comfortable rolling when you pop your ollie. Then you will have enough balance to land the ollie and roll away. The moving ollie should be your goal.
But first you need to learn to pop the ollie. There are a few ways to do an ollie. When just learning and really tweaking the ollie for high and or long gaps.
The tweak is the key to a good ollie but not the key to getting in the air. Getting into the air should be the first focus when learning the ollie.