INGLÉS DE NEGOCIOS: "THE NITTY-GRITTY"
When I hear a native speaker use an expression like this with a non-native English learner, I always groan because it is one of those phrases that, while it is very intuitive for native speakers, is difficult for a foreigner to understand.
The "nitty-gritty" is often used in a business context to talk about "the finer details". You might hear: "Let's get down to the nitty-gritty" which means that that person wants to talk about the details rather than a general overview.
Hope that helps!
Soy el director de Daniel Smith + Partners. Tengo más de 10 años de experiencia enseñando inglés a españoles.