Some human behavior is self-interested: when choosing which new shoes to buy, you might well focus solely on how different choices will affect you.
Psychological egoism is the view that all human behavior is ultimately self-interested: that everything you do is ultimately for your own benefit. Psychological egoists allow that we sometimes help others, but claim that this happens only when we believe that such behavior will ultimately benefit ourselves. For example, you might be polite and nice to someone but only because you hope they’ll later do you a favor.