Axial elongation is an increase in the axial length of the eyeball. In young, far-sighted people, this shifts them from hyperopia towards emmetropia. In adults, this results in a higher degree of myopia, which is exactly what we're trying to avoid!
Axial elongation in a lens-induced myopia context is primarily caused by hyperopic defocus, typically close-up activity while wearing glasses that are too powerful for the distance in question.
It can also be induced by using a lot of accommodation for a large fraction of the time, excessive myopic defocus (don't reduce too much!), or form deprivation from things like diffusers.