Cathexis generally means the process of investing someone or thing with emotional energy.

For example, you might cathexize your T-- ie invest him the energy that was once invested in your parents, but which became blocked because of various prohibitions or repressions that you underwent in order to adapt to whatever situation you were in. (Freud was less interested in the actual situation, theoretically at least, than the modes of repression or adaptation, the the drives underwent.)

So perhaps in order to unrepress or regain mastery of a situation, you might regress to some earlier stage of drive development (or you might be stuck at one, which would manifest itself in relation to your T). This would be analyzed ie interpreted within the analytic frame, until it gave way to a more 'mature" phase

Or you can cathexize objects, or parts of people-- etc.

Vautrin