Niobe (Nye-OH-bee) was a woman in Greece - in Thebes - a long long time ago (in the story). She was the daughter of Tantalus, and so she lived under a curse. Niobe had seven children, boys and girls, who were called the Niobids, meaning children of Niobe (I don't know what happened to their dad; he is not in the story). Niobe was very proud of her children, and she was proud that she had so many children. Too proud: she became hubristic (arrogant). She bragged that she had seven children, while the goddess Leto (LEE-toe) only had two (Apollo and Artemis). So Niobe thought she was better than Leto.
But it is very dangerous to brag that you are better
than the gods. Leto heard about Niobe and sent her two children, Apollo
and Artemis, to show Niobe who was really
better. Apollo and Artemis, who are both good marksmen, shot every one
of the Niobids with their arrows
and killed them. As Niobe stood there crying over her dead children,
Leto came and said, "Now who has more children?"
No matter how skilled people are, they are never any match for the gods. People need to remember their place, and not try to be stronger or wiser or smarter than the gods, or bad things will happen to them. Hubris also plays a big part in the Iliad
Learn by doing: find examples of hubris in modern television
More about hubris
D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire (Look under Artemis).
(Sorry, the only scholarly books I know of about Niobe are in German).