Any time you create a new Collection it will default to Private — meaning only you can see it and use it to cite resources on a Record or add resources to a Challenge.
If you want to contribute a Collection to a Challenge (or just want others to be able to see it in Explore) you can edit to “Make Public." Just note that you cannot revert it back to Private once you take this action!
Note: As of June 2019, Albums and Crates have all become Collections. Any Albums you may have created in the past are now Public Collections. If you had created any Crates, they are now Private Collections. All your Collections can be found HERE.