A commonplace book is a personal knowledge repository — a place to collect ideas, quotes, and observations that matter to you.
The practice dates back to ancient Greece. Renaissance scholars kept them obsessively. John Locke invented an indexing system for his.
The key insight: writing forces understanding. Copying a quote isn't enough. You have to put it in your own words to own it.
See also: how-i-take-notes, on-linking-ideas