A nit measures emitted light, not perceived brightness. That distinction matters, because how bright a display looks depends entirely on how much light is falling on it. The same 1000 nit panel is dazzling in a dim room and invisible behind a shop window at midday.
You will also see the unit written as cd/m² — candela per square metre. It is exactly the same measurement. Panel datasheets tend to say nit; outdoor and architectural specifications tend to say cd/m². If a supplier quotes one and a competitor quotes the other, you can compare them directly.
For reference: a laptop screen is 250 to 400 nit, a phone in bright sunlight mode reaches about 1000 nit, and a display that stays readable behind glass in direct sun needs 4000 to 5000 nit.