When someone only thinks about themselves and doesn't care about other people.
Selfishness is usually considered bad because it means someone is not kind to others.
Examples:
Her selfishness makes her take all the toys for herself.
I don't like playing with him because of his selfishness.
His selfishness means he never shares his snacks.
The cat's selfishness is not sharing the milk with the dog.
Her selfishness is so big, she never lets anyone else play with her toys.
The selfishness of the bear is why he doesn't want to share the honey.