The Omni is a device that is similar in appearance to a gold pocket watch. With its cover open, it consists of a small globe surrounded by several dials that allow a person to travel through time.
At the top left of the Omni is a red light. That light indicates that there is a problem with history that must be corrected. The red light doesn't stop them from traveling through time though. It just means that there is a problem. At the top right is a green light. That light means history is okay.
The Omni literally drops the Voyagers into history. When they arrive, a whistling sound can be heard, and then they drop out of the sky.