Original: To ensure that the Space Pirates can never obtain any more Metroids, the Galactic Federation sends several teams to the Metroids' home planet, SR388, to destroy them once and for all. However, when none of the teams survive, the Galactic Federation contracts Samus to finish the mission.
Simplified: Samus Aran goes to a planet called SR388 when the Galactic Federation told her to exterminate the Metroids that live on it.