If your hardwood floor has gotten wet and not dried quickly or something that was wet for a long period of time sat on it (such as an often watered plant on a terracotta saucer) you have probably discovered a mold stain on the floor. Here is how to remove it!
First, wash the floor with hardwood floor soap and water, then dry well, using a fan if the temperature is colder. Using a scraper (available at any building supply store), gently scrape just the top surface layer from the floor, making sure you are scraping with the grain of the wood.
Next, use a 180 grit sanding sponge and remove the layer of mold. If the stain is severe, you may need a small amount of dilluted bleach, but do be aware this may stain the hardwood… you may find tiny spots of mold stain are far less noticeable than potential bleach damage.
Next, use 200 grit sanding sponge to smooth the surface again, then refinish the hardwood, either with the same finish or with one that has been color matched for you.