Collections are essentially database tables where all of your information is stored.
From the datastore class, you can get a reference to a collection in order to execute a query or insert new documents into it.
// Get reference to a collection var collection = datastore.collection(name);
It is important to note here that it is not required for a collection to pre exist in order to get reference to it. A collection automatically gets created when you insert first document into it. You can execute multiple operations on a collection including insert, delete, update and search(filter).
To read more about them, click here.