It describes a person who is concerned primarily with their own needs and interests.
Here are some ways you could use "selfish" in a sentence:
* "He was being selfish by refusing to share his toys."
* "Her selfish actions hurt the people around her."
To use a noun related to selfishness, you could use words like:
* Selfishness: "His selfishness is a major problem in the relationship."
* Self-centeredness: "Her self-centeredness makes it difficult for her to connect with others."