A filter wheel holds several filters and rotates the chosen one into the light path without anything being unscrewed in the dark. It is an imaging accessory, and understanding why it exists explains a good deal about how imaging differs from looking.
The problem it solves
Monochrome imaging takes separate exposures through red, green, blue and narrowband filters and combines them afterwards. Swapping filters by hand between exposures means touching the instrument, which shifts the framing and the focus, and doing it repeatedly through a night is both tedious and error prone. A wheel removes the touching and can be driven by the capture software.
Why visual observers rarely need one
A visual observer uses at most two filters in a session and screws them into the eyepiece by hand, which takes seconds. The framing does not need to be repeatable to the pixel and nothing is being stacked afterwards. So the wheel solves a problem that only exists once exposures are being combined across hours.
What to check before buying one
Back focus, which is the distance available between the telescope and the sensor. A wheel takes up some of it, and if the instrument has too little the camera will not reach focus at all. This is the commonest frustration with a first imaging train and it is arithmetic that can be done before ordering anything.
Questions people ask about telescope
What is a filter wheel?
A rotating holder for several imaging filters, letting the capture software change filters without anyone touching the telescope.
Do I need a filter wheel?
Only for monochrome imaging with multiple filters. Visual observers and one-shot colour cameras do not.
What are telescope filters for?
Visually, to improve contrast on specific targets or to dim the moon. Photographically, to separate colour channels and to isolate emission lines.