What's a nifty-fifty?

It's a budget level 50mm prime lens. Pretty much all of the various camera makers offer one. There are several reasons that make them so nifty.

• Price - They're some of the most inexpensive prime lenses¹ available for most systems.
• Performance - They can compete optically at 50mm with zoom lenses costing much more.
• Maximum aperture - most 'nifty fifties' have a maximum aperture of around f/1.8. In comparison to most cheap consumer zoom lenses, that's about three stops wider/faster than a typical 18-55mm f/3.5-5.6 when set at 50mm.
• Mature optical design - The 'nifty fifty' is not new. It's been around for a long time. The design of 50mm lenses for cameras with diagonals of around 44mm (please see note ²) is quite mature.
• Simple optical design - 45-50mm is an optimal focal length for 35mm cameras with 44mm diagonal frame sizes and registration distances of around 44mm. That makes it what is known as a normal lens. Designing such a lens for such a camera system is much simpler than designing wider angle lenses or telephoto lenses for the same film/sensor size and registration distance³. As the focal length moves in either direction away from the diagonal of the format, lenses with the same optical quality as the normal lens become harder to design and more expensive to make.
• Relatively compact design - They're small and lightweight compared to many other lenses. The Canon EF 50mm f/1.8 STM (Canon's current nifty fifty) is 5.6 ounces and about two inches long. The Canon EF 50mm f/1.2 L is 19.2 ounces and almost three inches long.
• Value - For the price it is hard to beat the optical quality provided by most 'nifty fifties'. In terms of image quality "bang for the buck" most 'nifty fifty' lenses from the various manufacturer are some of the best values around. But they are far from some of the best prime lenses around. Perhaps one reason so many photographers claim the 'nifty fifty' lenses are super sharp is because they're the only prime lenses they've ever used and they are usually better optically than their consumer grade zoom lenses set at 50mm.

¹ A prime lens is a lens with only one focal length. No zoom.
² 35mm film has a width of 36mm and a height of 24mm for a diagonal of between 43mm and 44mm.
³ The distance from the image plane to the lens mounting flange. Sometimes called the flange focal distance.