selfish ˈsɛlfɪʃ/ adjective (of a person, action, or motive) lacking consideration for other people; concerned chiefly with one's own personal profit or pleasure. "I joined them for selfish reasons"
It is when you put your own needs infront of someone else's needs. Example, a friend asks for one candy from your candy bag and you say "No". That would be an act of selfishness.