Brightness is a lot more complicated than it would seem. Reflective light, direct light, and distance all get measured differently
A nit (nt) is a unit that represents the intensity of visible light. Nits are commonly used to describe the brightness of video displays, such as LED panels. The name nit is believed to have come from the Latin nitere, a word meaning 'to shine'. One nit is equal to one candela per square meter (1 nt = 1 cd/m2).
You may have seen light measures like candelas, lumens, lux, and others. But the nit, because it is a measurement of luminous intensity over a given area, makes it more useful for comparing relative brightnesses of displays even if they're not all the exact same size. It is the measurement you are going to see most.
Another common measurement of light intensity, the lumen, is often used in rating projectors, so you may run across it. It is a measurement of reflected light at a distance from the reflecting surface. And because projectors are still often used in indoor venues, comparing nits and lumens can be useful. In round terms, 1 nit is equal 3.5 lumens (3.426 to be more exact).
A simple way to remember the difference between nits and lumens is that the nit is analogous to sunlight, direct light; while the lumen is more like moonlight, or reflected light. Here is a table of common lumen and nit values for comparison:
As a rule of thumb, for an outdoor display, you need an LED screen producing something more than 5000 nits to appear luminous when in bright sunlight. A quality LED panel can produce 10,000 nits or more.