It stands for ‘Practical Object for Emotional Attachment’ and is a psychological phenomenon where people tie emotions to objects, so they feel better about buying these products.
An example would be sports teams fans that wear their team’s jersey or hat all the time because they find it comforting. Just like many people feel like there’s something wrong with them if they don’t have an animal in their life, some individuals think something is wrong with them if they don’t have a keychain to show off. They’ll buy one just to get that feeling of fulfillment. The keychain becomes a constant reminder of who you are and what you care about (just like wearing your college sweatshirt).