This applies to liquid-vapor equilibrium at pressures sufficiently low for the gas phase to obey PV = RT and where the activity coefficient of each component in the liquid phase is unity, i.e., it is an ideal mixture.
For such a liquid mixture, the total pressure at temperature T is:
where 
is the mole fraction and 
is the Vapor Pressure of pure component i at temperature T. Thus, for a binary mixture, if both liquid and vapor phases are present simultaneously and in equilibrium with one another, the pressure varies linearly with mole fraction from the vapor pressure of pure component 1 on one side to that of pure component 2 on the other.
Real mixtures obey Raoult's Law only as an approximation. The necessity for the pressure to be low restricts the range of applicability for any given mixture, but the requirement for the liquid to be essentially ideal restricts its use to mixtures of very similar molecules.