A 'second brain' is a personal knowledge management system — a digital archive of notes, ideas, articles and references that you can search later, like an external memory. Apps like Notion, Obsidian and Roam Research market themselves around the idea. Popularised by author Tiago Forte's 2022 book 'Building a Second Brain'. If your teen mentions building one, they are organising their study or creative work with care. Worthwhile habit; nothing concerning.