Formally it can be stated as dP over dT equals delta S over delta V, which gives the P-T trajectory of equilibrium states for a reaction as a function of the entropy and volume changes.  
Simply put it allows us, given one point on a coexistence line, to discover other points on that line.  Formally it can be stated as dP over dT equals delta S over delta V, which gives the P-T trajectory of equilibrium states for a reaction as a function of the entropy and volume changes.  FTP, identify this equation that explains the inexact relationship of the temperature of a liquid and its vapor pressure and which is named for its two discoverers.
Simply put it allows us, given one point on a coexistence line, to discover other points on that line.  
Clausius-Clapeyron equation