As apps get bigger, it is important to keep track of what the different pages, key actions, styles and data types are about. This will make modifying the app down the line easier, and will help other people collaborate with you on the application simpler.
Bubble has a way to comment on most objects. Whenever you see a quote icon (in the top bar of the Property Editor), clicking on it will reveal the Comment Panel.
When the quote icon is filled, that means the object has been commented already.
The comment panel also lets you view all comments as one, and clicking on an entry will show and comment and take you to the element in the editor. Many users use this as a way to collaborate and set up a todo list on their app.