Myelin is a fatty insulating layer that wraps around the axons of frequently-used neural pathways, dramatically increasing the speed and reliability of signal transmission along that specific circuit — in some cases increasing conduction speed by a factor of 100 compared to an unmyelinated axon. Repeated, focused practice of a skill increases myelination along the specific neural circuits involved in that skill, which is a large part of the physical mechanism behind a practiced skill eventually feeling fast and automatic rather than effortful and slow.
This has a practical implication for how practice should be structured: myelination responds to focused, attentive repetition of a specific pattern, not passive or distracted repetition — which is part of why 'practicing on autopilot' produces much weaker skill gains than the same number of repetitions done with full, deliberate attention on getting the pattern right each time.
Bengtsson's team used diffusion tensor MRI — which can measure the integrity of the brain's white matter, largely made up of myelinated axon bundles — to compare professional pianists against non-musicians, and separately tracked children during piano training over time. The amount of practice correlated directly with white-matter structure in the specific tracts connecting brain regions involved in the fine motor control piano playing requires, and the correlation was strongest for practice done in childhood and adolescence, when white matter is still actively developing, though changes were detectable in adult practicers too.