Collapsing bubbles can damage the impeller and volute. That makes cavitation a problem for both the pump and the mechanical seal. Vapor pressure is about liquids boiling. If you know the temperature of your product you need to know its vapor pressure to prevent boiling and the formation of bubbles. In the charts section you will find a typical vapor pressure chart for several common liquids.
Specific gravity is about the weight of the fluid.
If the fluid floats on this fresh water its specific gravity is less than one. If the fluid sinks in this water the specific gravity of the fluid is greater than one. If you cannot do this have someone show you where they are located. Liquid velocity is another import concept. If the velocity decreases, the pressure increases.