A guide scope is a small telescope riding alongside the main one, watching a single star and reporting its movement to software that nudges the mount. It exists to fix the last few arcseconds of tracking error, which matters only once exposures run into minutes.
What autoguiding actually corrects
Even a good mount drifts: gear imperfection produces a slow periodic wander, polar misalignment produces steady drift, and flexure adds its own. Over thirty seconds none of that matters; over five minutes it turns stars into short lines. Autoguiding measures the error continuously and corrects it before it becomes visible.
Guide scope or off axis guider
A guide scope is simple and cheap and can suffer differential flexure, where the guider and the main telescope move slightly relative to each other and the correction is wrong. An off axis guider takes its star from the main telescope's own light path, which removes that problem and is fiddlier to set up. Long focal lengths usually need the latter.
When you do not need one
Visual observing, always. Lunar and planetary imaging, always, because exposures are milliseconds. Wide field imaging with short exposures on a well aligned mount, often. Guiding becomes necessary when exposures are long enough that the mount's own error shows, which is usually beyond a minute or two.
What it costs to add
A small guide scope, a guide camera and a laptop or dedicated controller. It is one of the cheaper steps in an imaging setup and it is often the step that turns a frustrating one into a working one, because tracking error rather than optics is what limits most first attempts.
Questions people ask about guide scope
What is a guide scope?
A small telescope alongside the main one, feeding a camera that watches a star so software can correct the mount's tracking errors.
Do I need a guide scope?
Only for long exposure deep sky imaging. Visual observing and planetary imaging never need one.
Guide scope or off axis guider?
A guide scope for short focal lengths, an off axis guider for long ones where differential flexure would spoil the correction.