Your commonplace book is just a notebook where you collect quotes from things you’re reading, observations, passing thoughts – in short, anything that catches your attention that you want to remember.
You can use it for one specific purpose, if you are focused on one project, or you can make it a mish-mash of everything. Some people leave room at the beginning to keep an index of categories, and some – I include myself in this group – just throw things in randomly.
Taking notes physically has a different effect on the brain, and aids with recall and processing of information in a way that digital note taking does not (see this piece from Harvard), so it’s likely that just by picking up a pen and writing something down, we stand a better chance of remembering what it was we found so fascinating anyway.