Center of gravity
For a plane to fly well it needs to be balanced and that balancing point is just back from the leading edge of the wing which is usually the thickest part of the wing
(this is just a general rule, it becomes a bit more complicated with jets as the whole aircraft is designed as a wing)
It has to be balanced in its flying state so you must make sure the battery is in place. It doesn't have to be exactly balanced you just have to make sure its fairly close. You can test how close you are when you start flying by doing a dive test
