Newcomb's Problem is a thought experiment in Decision Theory exploring problems posed by having other agents in the environment who can predict your actions.
The mysterious alien superintelligence Omega has offered you a choice. There are two boxes, A and B. You can take both boxes, or only take box B. Box A definitely contains $1000. If Omega predicted that you would only take box B, it filled box B with $1,000,000. If it predicted that you would take both boxes, it left box B empty.
Omega has played this game many times before, and its predictions have almost never been wrong. Omega has already made its prediction and filled or not filled box B accordingly. Do you take both boxes, or only box B?