The talisman of Troy
The Palladium was a small image of Athena, traditionally said to have fallen from the sky, on which the safety of Troy depended. As long as it remained in the city, the city could not be taken. The story explains why the Greeks needed to steal it before the sack.
The Iliad does not mention the Palladium. It does show the Trojans honouring Athena and the Greeks trying to win her favour, but the specific talisman belongs to the later mythological and antiquarian tradition.
The theft
Later sources, including the Little Iliad and the Roman poets, tell how Odysseus and Diomedes entered Troy in disguise and carried off the Palladium. The episode was popular in art and drama, and it became one of the standard episodes of the sack narrative.
In this wiki the Palladium is included as a traditional object that explains the fall of Troy in later storytelling. It is not part of the Homeric poems, and the distinction matters for reading them: the Iliad’s Troy falls because of human decisions and divine will, not because a talisman was stolen.