Vapor pressure is the physical pressure created by molecules escaping a liquid into the air. Pure liquid water has a specific, measurable vapor pressure at normal room temperature. If you dissolve solid salt into that water, the vapor pressure drops noticeably.
Raoult’s Law explains exactly how much that pressure drops using simple math. The dissolved salt particles take up physical space at the surface of the liquid. This physically blocks some liquid water molecules from escaping up into the air.
Because fewer water molecules escape the liquid, the overall vapor pressure goes down. This scientific law relies heavily on a concept called the mole fraction. The mole fraction is simply the percentage of molecules that belong to the solvent.
A common student misconception is that the type of dissolved chemical matters. In a perfectly ideal solution, only the total number of dissolved particles actually matters. This concentration-dependent behavior is known in chemistry as a colligative property.
This vapor pressure drop is responsible for other important everyday physical effects. It is the real reason salt water boils at a higher temperature than pure water. It also completely explains why salt water freezes at a much lower temperature.
Real world solutions rarely follow this mathematical law perfectly in the lab. They deviate because different molecules attract or repel each other in unique ways.