The nighttime adjustment is applied on top of the upwind/ downwind adjustment and allows the router to take into account that you sail the boat slower / with less sail at night for safety reasons. It is a percentage factor, so if you use the value 70%, this tells the router you expect to sail 30% slower than the base polar you have set. This value will be added on top of any other polar speed adjustments you have applied.
Some other relevant information relates to the definition of day/ night:
"There are different definitions for sunrise/set: 'official', 'civil', 'nautical', and 'astronomical'. Some references if you like:
What is used for the router is the nautical definition, which should correspond to when you have daylight enough to use for navigation.