Safety Not Guaranteed
From Halopedia, the Halo wiki
Safety Not Guaranteed is an achievement in Halo: The Master Chief Collection. This achievement is obtained by completing one of the two Halo 3: ODST LASO Campaign playlists, "LASO Non Stop Action" or "SLASO Campaign".
The achievement's name is in reference toasking for a volunteer to time travel with them, warning them "Safety not guaranteed."