Basically, all the things you put in your bullet journal can be classified into different topics, which are called collections. Think of collections as any tasks, notes, or pages that are related to each other.
For example, let’s say you have a few pages that you use to keep track of everything in January. The topic of this collection is January or January logs, and you can list it that way (along with the relevant page numbers) in your bullet journal’s index.
When listing a new collection in the index, just write the collection title (for example, January Logs) followed by the page numbers (24-28).
Lumping pages into collections can help you organize and keep track of notes for goals, habits, courses you’re taking, and any ongoing projects you have.
Sometimes your collections get separated by a few pages in your bullet journal. In order to make it easier to find all the pages of a collection (without having to go back to the index again and again), we use a technique called threading, which is simply noting the other page numbers of a collection.
So, for example, if you have a collection on page 21-22, and it continues on page 30, you can make a note near the bottom of page 22 to indicate that the collection is continued on page 30.