I use the term “meridian flip” quite often without explaining it well, and the term isn’t exactly intuitive.
The telescope follows the sky by running a motor on one of its axes, called the right ascension axis. If you have the mount aligned with the celestial pole properly, which is very near Polaris for those of us in the northern hemisphere, you only need to turn this one motor to follow something in the sky as it moves from east to west with the rotation of the earth.
The most common amateur imaging mount type, the German equatorial mount, is generally held up by a tripod or pier. In certain positions, if the telescope were to track something from the east to west indefinitely, the telescope would strike the pier. To keep this from happening, when a target is directly overhead, or at the meridian, the telescope flips to the other side of the pier before it continues to track the object. This keeps the telescope above the pier as it continues tracking west, and so there is no worry of damage.
Here is a video of my telescope performing a meridian flip in real time.
The telescope tracks the sky from left to right in the video, as in the timelapse video here. That link contains a video that shows two meridians flips for two targets throughout the course of a night.
