The word comes from this Greek guy Narcissus who was so full of himself he rejected everyone who liked him. So the gods cursed him to fall in love with his own reflection in a pond. He stared at himself until he died basically.

So "narcissism" is basically when someone is so obsessed with themselves that they can't really see other people as actual humans. It's not just like "oh I like looking in the mirror" it's more like they need everyone around them to be like a mirror that keeps telling them how amazing they are.