Nearly all of our knowledge about the early universe comes from the observable signatures of two phase transitions: recombination at z ~ 1100 and the late stages of cosmic reionization at z ~ 6. Due to its tenuous ionosphere and shielding from terrestrial radio interference, the lunar farside is unparalleled as a site for conducting observations of the intervening billion years, in which the first stars, black holes, and galaxies form. Astrophysical processes are expected to produce three inflection points in the sky-averaged 21-cm spectrum prior to the completion of reionization, though interpreting these features will be challenging due to degeneracies among model parameters. In this presentation, I will describe how observable measures of the 21-cm sky translate to physical properties of the Universe, and most importantly, how these diagnostics can be used to constrain the properties of the first stars and black holes. Future missions like the Dark Ages Radio Explorer, uniquely situated above the radio-quiet lunar farside, have the best chance of constraining cosmic evolution at z > 15, which I will highlight as the most straightforward part of the 21-cm signal to decipher.