A two-level system (2LS) is probably the simplest, most fundamental object one can do stuff with: it takes two values, say, 0 and 1 (then it's a bit) or two states of something, say ground and excited state (as is typically understood in physics).
I do study 2LSs a lot. Whenever I study something, I eventually come back to the 2LS. It's only a matter of how much time it takes to get back home.