75-300mm refers to the zoom range: the difference between the shortest focal length the lens will go to and the longest . 75mm is a little over the "standard" length of, usually, 50mm, which means you'll get a slightly narrower angle of view - or, to put it another way, the lens will slightly magnify the image. 300mm is a fairly long telephoto and will fill the viewfinder with a distant object tthat, with a 50mm lens, would only be a tiny part of the image. 4.5 - 5.6 refers to the maximum possible aperture, which changes as you zoom.The minimum aperture will probably be f/16 or f/22. The smaller the number, the bigger the hole through which light passes. The bigger the hole, the more light gets through so the faster the shutter speed you can use in the same light level. But the aperture also affects the "depth of field": the distance range over which your image will be in focus. The larger the aperture, the smaller theh depth of field. balancing all these things to get the kind of image you want is an important part of photography. It's a good lens, by the way.