Overcoming agoraphobia is to be taken in small steps. Maybe venturing outdoors is to stock up on snacks when her parents and sister forgot to get her some.
(I feel like the guy who goes through explaining all the jokes so they aren't funny anymore at this point but...)
This isn't an image of a literal event. Consider it a dream sequence or something.
All of the people around her are moving onwards with their lives, but she's completely stuck where she is in life. (The girl on the bottom right even looks like her younger sister, when she was in Kindergarten.)
