When proving that a problem is NP-hard, we usually reduce another problem, already known to be NP-hard, to out problem. This process must start somewhere - there must be a first problem which is proven to be NP-hard. For this first problem, say SAT, we need to show that every problem in NP can be reduced to SAT. This is the master reduction, which reduces an arbitrary problem in NP to SAT. Following that, we use reductions that reduce a single problem to the problem at hand.
The name master reduction is perhaps common in Germany, but I have never heard of it. In any case it's just an informal name. Just like the fundamental theorem of algebra is some theorem that somebody at some point decided to call that way, so master reduction is just a term that somebody invented for some reason. I have tried to explain the significance of the term above, but in reality there is no higher significance to the term beyond the particular object it refers to.