The intricate details of a situation or process. For example, It takes a newcomer some time to learn the ins and outs of the legislative process, or David really knows the ins and outs of how this engine works. This usage alludes to the tortuous windings and turnings of a road or path.
[ Second half of 1600s
Those with position and influence and those without, especially those in office versus those who are not, as in “Juan stood well both with Ins and Outs” (Byron, Don Juan, 1823).