Defense mechanisms are how the ego deals with stress from being the mediator between the id and super ego. Displacement specifically, is when we channel an unacceptable impulse to a more acceptable target.
When I was younger I would throw temper tantrums because my parents would take away my toys. I was mad at my parents, but I couldn't necessarily express that out loud.
The id is the part of the mind that is selfish and has aggressive tendencies. The id wants what it wants now. The id is in conflict with the super ego but we are only aware of the ego's decision.
When my sister was first born my id would take over and not want my sister to play with any of my toys. She wasn't allowed to play with any of the toys because they were all mine, my id was more in control of my unconscious.