The nit is a unit of measurement for brightness. Actually, it's technically a measure of luminance. "Luminance" refers to the intensity of light, an objective property of nature. "Brightness" refers to how we perceive luminance. Brightness is nonlinear and subjective, so it can't be measured. But I'm nitpicking. (Ha! Nitpicking. ... Anyone?)
So, nits refers to luminance. The more nits, the
brighter more luminescent the screen. The best consumer laptops on the market right now reach about 300 nits. The iPhone 6 maxes out at 500 nits. The latest models of the Toughbook CF-19 and the Toughbook H2 reach up to 6000 nits in direct sunlight due to "TransflectivePlus" and Panasonic CircuLumin display technologies.
Higher nit screens are easier to view in a broader range of lighting conditions. It can be hard to read your smartphone outside on a sunny day. You won't have that problem with your Toughbook H2.
OK, but what is a nit?
"Nit" — probably derived from the Latin nitere, to shine — is a widely accepted slang term for candela per square meter, cd/m². As it turns out, physicists and engineers have slang, and they derive it from Latin.
One candela is, for all intents and purposes, the same thing as one candlepower. Though the terminology has been updated, it is still a measurement of the approximate amount of light emitted from a 19th century whale oil candle.
Take a birthday cake measuring one square meter and stick six thousand lit candles in it. That's about how bright the latest Toughbook displays can get. Unlike the ridiculous hypothetical birthday cake, a Toughbook probably won't burn your house down.