A hydrometer is used for testing the specific gravity (SG) of what you are fermenting. You do this by taking a sample out through the tap of your fermented product in to a test tube. Remove your airlock first so that the water is not sucked into your fermenter. Give the hydrometer a spin and wait for it to stop moving. The spinning is to remove any bubbles that could be stuck to the sides of the hydrometer, thus giving you a false reading. Always follow the guidelines on the yeast you are using and be very aware of the different results you will get from different fermentation temperatures. See the other temperature related questions on this FAQ page.  The float line is your reading and you are looking for a stable reading two days in a row. This means fermentation is finished if the process has been done at a suitable temperature. Now you’re ready for the next step, bottling, kegging or distilling.