The "SVP" is a function of temperature, and is tabulated at various temperatures. Of course, at water's boiling point under standard conditions, the "SVP" is "1 atmosphere".
When you collect a gas by bubbling it thru water to a graduated cylinder, this gas is saturated with water vapour.
Thus P_"laboratory"=P_"gas"+P_"SVP".
At 25 ""^@C, "SVP"="23.8 mm Hg". So you have to subtract this "SVP" from the laboratory pressure in order to find P_"gas", the pressure exerted by whatever gas you are collecting.
"SVP" is commonly reported in "mm Hg" because the mercury manometer is still the chemists' means of measuring pressure. Clearly, you don't need to memorize these values; they should be provided in an exam.
