Preflower is a state the plant goes through before it begins to truly flower. The junction where the large fan leaves meet the main stem will begin to develope new sets of branches that will later turn into the clone sites. When you see these new branches starting to form the plant will soon show its sex at the vee formed by this new growth.
If the plant is left in the veg state under a 18/6 light cycle most will begin to show signs of preflowering somewhere around 4 to 8 weeks. If the plant is placed into flower or 12/12 light cycle before it has reached the preflower stage it may take anywhere from a couple of days to a couple of weeks for the plant to go into preflower then into true flower.
Think of preflowering this way. A mother plant is a plant that is held in the preflower stage until clones are taken or the plant is placed into flower. This is why you can put a clone directly into flower as soon as it has rooted. It is already in the preflower stage the same as the mother.