Answer:
An analogy is comparing two things. One situation is on one side of the double colon (::), and another is on the other side. Usually (if you're in school, anyway), you'll have to find the relation from the two situations and find the missing word. For example,
sky:blue::grass:green (the sky is the color blue, the grass is the color green).
fish:gills::human:lungs (a fish uses gills to breathe, a human uses lungs).
earth:sun::electrons::nucleus (earth revolves around the sun, electrons around the nucleus).
glove:hand::sock:foot (you wear a glove on your hand like you wear a sock on your foot).
unintelligent:smart::fast:slow (opposites)
shark:ocean::camel:desert (where the animals live)