F/4.5 1/1250s iso100? What does that means? Nikon DSLR?

Please correct me and educate me! Thanks!

f/4.5 << What that?

1/1250s << Is this the shutter speed?

iso100 << I already know what ISO means. The lower the lights, the higher the ISO right?

First question:
F/4.5 is a moderate size lens (aperture) opening. The smaller the number (eg: 2.8) the larger the opening and the "faster" the lens is considered. On the opposite end, many lenses can "shut down" to as small as f/22.

Second question:
1/250s means that the camera's shutter would be open for 1/250 of a second. A higher number (eg: 1/500) means the shutter is open for a shorter time and is considered a "faster" shutterspeed. A lower number (eg: 1/60) means the shutter is open for a shorter time. Faster shutter speeds are best for capturing motion or action. Lower shutter speeds work best in low light.

Now, there's a delicate balance between shutter speed, aperture, and ISO. To achieve the same amount of light on the camera's sensor, you'd have to adjust one area up, if the other area(s) go down, and vice versa. Think of it like a garden hose size, water pressure, and time you leave the water running. As each element changes, you'll need different numbers of the other components to fill the same size bucket. If the bucket situation changes, so will your setting to find the perfect balance.

Shooting photos is the same. Find the balance points, after deciding which of the 3 elements is most critical for your composition. For example, if you are capturing sports, you'll concentrate on a higher shutter speed. If you are taking portraits, then a larger lens opening will give you better bokeh, where the foreground and background are out of focus, but the subject is perfect. If the lighting situation is fairly dark, then a higher ISO might be called for.

Experience is the best teacher here.

If you'll read the following and take time to learn about the Exposure Triangle - Aperture/ISO/Shutter Speed - it will help your photography:
http://www.digital-photography-school.com/learning-exposure-in-digital-photography

Aperture:
Inside the lens there's a diaphragm which consists of a number of movable blades which can open or close to create an opening called the aperture or f-stop. Lenses used to have these aperture/f-stop values on the lens barrel and they often looked like this: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 with some lenses going to 22 and 32. Because the older lenses were mechanical those were the aperture/f-stop values available. The smaller the number the bigger the opening of the diaphragm and the larger the number the smaller the opening. Modern lenses usually lack this numbering system because they are controlled electronically through the camera and the size of the opening formed by the movable blades of the diaphragm is step-less, i.e., almost infinitely variable within limits. A modern lens is thus capable of an aperture/f-stop such as f4.5 or f7.2 or f9.5.

Shutter Speed:
The shutter speed controls how long the light admitted by the aperture/f-stop chosen is allowed to expose your film or sensor based on the ISO used. In your question 1/250 is the shutter speed chosen by your camera (or you) to produce a correct exposure based on f4.5 when using ISO 100. If you changed to ISO 200 you'd then have two options: change the shutter speed to 1/500 and leave the aperture ar f4.5 or change the aperture to f9 and leave the shutter speed at 1/250. The shutter speed also determines how any motion in your picture is captured. A fast shutter speed such as 1/250 or 1/500 or 1/1000 will "stop" motion while a slower shutter speed such as 1/125 or 1/60 or 1/30 or 1/15 will show motion as a blur with slower and slower shutter speeds showing more blur.

ISO
Simply a measurement of the sensitivity to light of a light-sensitive surface whether film or digital sensor. The lower the ISO number (50, 100) the less sensitive. We usually use a low ISO outdoors on a sunny day. The higher the ISO number (200, 400, 800, etc.) the more sensitive. We usually use a higher ISO indoors when we don't want to use flash or on overcast or windy days. There's a trade-off however. The lower the ISO the better the image quality while the higher the ISO the lower the image quality - digital noise becomes apparent which causes a loss in detail.

So read the referenced link above and learn about the Exposure Triangle.