Cortical thickening

You know I studied neuroviology in college and I'm realizing I've never quite understood what this is.

I understand there is gray matter and white matter in the brain. So I asked Google if gray matter is neurons and then what is white matter. So white matter is like myelin and fat? Accents? So the gray matter is the neuron kids so to say, and a white matter are the cables together? So cortical thickening could be thought of as a mango of cables that got stuck on their way to their destination?

Does long-term potentiation affect the amount of white matter in the brain and if so how? Or would that result in an abundance of gray matter instead? I just don't understand how they interact and I feel very stupid for asking but I just don't get it.