Nits
In phones, this most commonly refers to the maximum potential brightness of the display panel. Brighter displays are easier to see in brighter environments such as outdoors in daylight.
Nit is an international standard for luminance (brightness) that is equivalent to one candela per square meter (cd/m2). Thus, it is technically a measure of light emitted per unit area.
Most phone displays automatically adjust overall brightness to compensate for darker and lighter viewing environments. Some phones also have a distinct "outdoor" mode that further boosts display brightness when outdoors in daylight. This may have a separate nits rating.
Some phones have a distinct rating of "peak" brightness in nits. This is a separate maximum brightness that the display can output on small parts of the display for short periods of time. (The whole display cannot output this peak brightness all at once.) In practice, this is implemented only when displaying HDR content.
See: HDR
