These are collections that people create based on their own interests and needs. If you are new to bullet journaling you may not know what a bullet journal collection actually is.
Simply put, a collection is just a collection of related ideas grouped together, like a bullet journal insert, and can take any form! It could be a list of things you want to do, places to visit, things to buy, things to try, notes on your future. The list goes on indefinitely. It can be anything!
Regarding the bullet journaling method, some collections are inherited from the system, such as your daily and monthly logs, and most people use them. Every log and entry in your bullet journal is a collection – because they are a collection of related things.