Put simply, projections are a way of representing collections. Using a very simple syntax, you can inspect very complex object models inside collections.

Imagine you have a collection of User objects. Each of these objects has a Parent. Now say you want to get a list of all names of the parents (assuming the Parent class has a name field) in the hierarchy of users, you would write something like this:

parentNames = (parent.name in users);

You can even perform nested operations. Imagine instead, that the User object had a collection member called familyMembers, and we wanted a list of all the family members names:

familyMembers = (name in (familyMembers in users));