You can have vapour when there is no liquid present and that vapour would exert a vapour pressure.
If however you have liquid and vapour present in dynamic equilibrium with one another then the pressure exerted by the vapour is the saturated vapour pressure.
So start off with a container with only vapour in it.
The vapour exerts a vapour pressure.
Now do something eg add liquid, cool the vapour, reduce the volume of the container, etc, so that there is also liquid in the container.
Then the pressure exerted by the vapour is the saturated vapour pressure.
The graph represents temperature vs. specific volume for a pure component. Because this is a pure component, several facts are known:
1) In order to gather the given data, some container was partially filled with a given liquid, with the remainder of the container filled with the vapor of the same substance. The container contained a substantial fraction of liquid and a substantial fraction of vapor (e.g., 50% liquid and 50% vapor), to ensure that some liquid and some vapor remained in the container as the liquid temperature was increased.
2) The liquid density goes down (liquid specific volume increases) as the liquid temperature increases.
3) There is a relationship between the temperature of the liquid and the pressure inside the container. This relationship is determined by the vapor pressure of the liquid, which can be calculated by the Antoine equation (see https://en.wikipedia.org/wiki/Antoine_equation). As the temperature inside the container goes up, the vapor pressure of the liquid goes up. As a result, more liquid goes into the vapor phase, and since this is a closed container, the pressure in the container goes up as a result. As the pressure increases, the density of the vapor phase goes up (vapor specific volume decreases).
4) At some point, as the temperature increases, the liquid density decreases enough to equal the vapor density, and the critical point is reached. At this point, there is only one phase in the container, as the liquid and vapor phase densities are equal.
Because increasing temperature causes liquid specific volume to increase, and at the same time causes vapor specific volume to decrease due to the dependencies between temperature, vapor pressure, and specific volume, the width of the saturated liquid-vapor line decreases as temperature increases, until you reach the critical temperature.
Best Answer
Because liquids are in general much less compressible than gases, meaning that a small reduction in volume requires a big increase in pressure.