Today I happened to be in the area where I grew up...
Today I happened to be in the area where I grew up so I went and checked out my old childhood home. I knocked and asked if I could look around a bit, but the owners said no and shut the door in my face.
This joke is based on the breaking of a false assumption on the part of the listener. The listener did not expect that the person that he visited was his parents (especially since they shut the door on his face.)
My parents can be so rude sometimes.
The punchline disabuses the listener of the notion, and that is funny.