Nuts and Bolts of Something

Reading technical books, we have seen a lot of the noun phrase "Nuts and Bolts of...". What does it address about?
nuts and bolts: the basic facts or important details of something.
Ex:
[1] "Ted will be an excellent trader", his millionaire grandfather said, "once he learns the nuts and bolts of the profession".
[2] In this chapter, we will present the nuts and bolts of Android.