Devaluation in psychology means that someone doesn't value or appreciate the things or people that they used to like or consider important. It's like when you have a toy that you really love and play with all the time, but as you grow up, you start to think it's not as cool anymore and you don't want to play with it.
The same thing can happen with people. If someone used to think highly of their friend or partner, but then they start to think they aren't as good as they used to be or don't appreciate them, that's devaluation. It can be harmful to the relationship because the person who is being devalued may feel hurt or unimportant.
Sometimes, devaluation happens because of our own insecurities or when we feel like we are not getting something we want out of the relationship. Other times, it may be because the person we used to value has done something to upset or disappoint us.
It's important to recognize when we may be devaluing someone and try to understand why we feel that way. It's also important to communicate with the person and express how we feel so they can understand and work to improve the relationship.