There is a class of small single-tube instruments sold under several names, and they are all the same idea: a telescope small enough to carry in a coat pocket, with the compromises that implies.
What a monocular actually is
A single small refracting telescope with an erecting system, typically 8x25 or 10x42, giving an upright image. It weighs a fraction of a binocular of the same specification and shows the same view through one eye. For occasional daylight use it is genuinely handy, and for extended viewing it is tiring.
Handheld spotting telescopes
Slightly larger single tubes, often zooming from 20x to 60x, sold for landscape and wildlife. Handheld they are unusable at the top of the range and adequate at the bottom, which is a way of saying they need the tripod that the marketing photographs never show.
Phone telescopes and adapters
A small monocular sold with a clamp for a phone camera. The optics are usually modest and the results are limited by the phone's small sensor, but they work, and a phone held to the eyepiece of any real telescope will do the same job better. If the aim is photographing what you see, an adapter for an instrument you already own beats a dedicated phone scope.
Questions people ask about telescope
Is a monocular telescope any good?
For pocketable daylight use, yes. It is less comfortable than binoculars for extended viewing and shows nothing more than the same specification in two barrels would.
What is a handheld telescope?
A small single-tube instrument, usually with an erecting prism so the image is the right way up, meant for daylight use without a tripod.
Do phone telescopes work?
They work, within the limits of small optics and a phone sensor. A phone adapter fitted to a real telescope gives better results for similar money.