Gravity readings are taken to gauge fermentation. You need to buy a hydrometer. Take a reading of your beer before you send it into the fermenter. To do so, fill the tube up with wort (the beer) until it's about 2-1/2" from the top of the tube. Insert hydrometer. Spin to release the bubbles and place on flat surface. Read the number that corresponds to the wort line. That is your OG, or original gravity. Think of it as a starting point. Your recipe will have an OG, that is your goal, to reach that number. If your number is lower than the OG on the recipe, your beer has less sugars than the recipe. Your efficiency is low. If your OG is higher than the recipe, you have more sugars that the recipe called for, your efficiency is high. The higher the OG, the more alcohol will be in your final product, for the most part.
So, you've got the OG, wort goes into the fermenter, in goes the yeast and after 2 weeks, you take a gravity reading with your hydrometer to see if the yeast is all done working. The number should be nearing your FG, or final gravity. The difference in numbers OG-FG, is the amount of sugars your yeast ate and turned into alcohol. If your beer remains at the same FG for 2 days, about 3 gravity readings, it's done, and ready to be bottled. You can use the OG-FG to measure ABV.