Here it is in simple terms.
In your brain you have cells called neurons that arrange in circuits. A substance called myelin gets wrapped around these neurons when you use the neuronal circuits in your brain. This process is called myelination.
In a little more detail:
Every human movement, thought, or feeling is a precisely timed electric signal traveling through a chain of neurons—a circuit of nerve fibers.
Myelin is the insulation that wraps these nerve fibers and increases signal strength, speed, and accuracy.
The more we fire a particular circuit, the more myelin optimizes that circuit, and the stronger, faster, and more fluent our movements and thoughts become.