ISO of 1500 is generally for low light/night photography.
ISO and shutter speed have to work together to allow enough light in to create the picture. The best example I've read is to think of the ISO number as bees in a beehive. At ISO 100 you have 100 bees building the picture. Takes them longer to do it because there are less of them so your shutter has to stay open longer for the bees to accomplish the picture. However, they do a better job because they are not all bumping into each other.
ISO 1500 gives you 1500 bees on the same size working area. They can do the job really fast because there are so many more of them. BUT, they tend to bump into each other a lot and this causes imperfections (grain or "noise"). Because there are so many more bees the shutter only has to be opened for a very brief moment.
The key for a beginner is learning the relationship between these three points. Aperture (how big the iris of you camera opens), ISO, and shutter speed. A good way to mess with these is to take a picture in auto and see what settings the camera chose. Now switch the camera over to "M"anual and duplicate the settings. Now start messing with one of the three and look at what it does to your picture. Then repeat with one of the other settings and so forth. Eventually you will start to understand each one and finally be able to put all three together so they cooperate with each other.