Until the 1960s the adult brain was believed to be fixed. Today we know synapses strengthen or weaken with use, new connections form, and new neurons appear in the adult hippocampus.
The popular phrasing — “neurons that fire together wire together”, attributed to Donald Hebb — is a simplification, but it captures the idea: repetition changes structure. A thinking pattern practised daily for ten years has a physical counterpart, and a new one has to be built, not merely decided on.
Two practical consequences follow, in opposite directions. The first is hopeful: patterns are not permanent and can change at any age. The second is sobering: change requires repetition, not understanding. An insight in one session does not rewrite ten years of practice — which is why effective therapies include work between sessions, and those that stop at conversation deliver less.
The word is also heavily used in marketing. That an exercise “changes the brain” says nothing about its usefulness: every activity changes the brain, including a useless one.