Nits are a level of brightness for a given level of surface area. If you like a bright monitor you can use the variable as a benchmark in the same way you gauge resolution in pixels per inch. On a sunny day (compare it to using a smartphone outside) brightness can be important.

I'm not super familiar with 'low' or 'high' brightness, but as far as I know 600 is very good. The iPhone 7 for example has about 700 nits max, for outdoor use. So it seems as if this is close to an outdoor-level monitor, which should perform well under any indoor condition, especially with some light shades if necessary.