When a non-volatile solute is dissolved in a volatile solvent, the vapour pressure of the solution:

The non-volatile solute occupies some of the surface that would otherwise emit solvent vapour, reducing the rate of vaporization. Hence $P_{solution} < P^0_{solvent}$ — Raoult's law gives $P = x_1 P_1^0$ with $x_1 < 1$.