Determining Saving Throw Values

So this may be a newbie question but I am currently DMing a 5e home campaign, and I was looking at the Drow monster stat block. They have innate spellcasting abilities and one of those is Faerie Fire. In the description it states that the targets must make a DEX saving throw or be outlined in color. Now my question is, how do I determine the number that is needed to PASS the dex saving throw as it is not stated and I am just a bit confused as to how to arrive at this number. Thank you guys so much!