ERH = Eutonic Relative Humidity.

Here E(A+B) is the optimal or Eutonic mixture. And L here refers to the liquid phase. So, to the left we have a mixture of A with E(A+B) and so only part of it is liquid above the Eutonic point, and the salt A will remain in its solid phase. To the right, similarly, salt B remains in its solid phase.

So as the humidity is increased, for a given A / B mixture, first the lower horizontal line is reached, at which point some of the mixture of salts becomes liquid. This is known as the "eutonic relative humidity" - the point at which any mixture will start to take up some water vapour. As humidity is raised further, more and more of the mixture becomes liquid. Eventually the upper, curved line is reached - and at that point, the entire mixture will be in its liquid phase.

Adapted from https://en.wikipedia.org/wiki/File:Strukturalnie_Eutektyka_brak_rozpuszczalnosc_w_ss.JPG