A CRC card contains three pieces of information:
The Class name
The class's main Responsibilities, described in human language using only a few words
The class's main Collaborators, that is, the names of other classes that this one talks to
A collaborator is a class that your class needs a reference to.
For example, a Player might spawn Missiles. But if the Player doesn't actually need to talk to Missiles, then it's not a collaborator.
When you think about the Collaborators, imagine it as a one-way arrow. What classes does this one talk to?
Sometimes one class needs to talk to another, but it's only in one direction. In that case, class A is a collaborator of class B.
Other times, the classes need to talk to each other. In that case, A is a collaborator of B, and B is also a collaborator of A.