ISO

ISO refers to the sensitivity of the camera sensor to light and along with aperture and shutter speed works together to produce an exposure. It is also represented by stops of light, 100, 200, 400 and so on. Lower ISO values makes your camera sensor less sensitive to light while higher ISO values makes your camera more sensitive to light. In a sunny day you can use an ISO minimum value of 50 or 100, 200 - 400 in an overcast day/ dim lighting and 800-above in night. However, you should keep in mind that by increasing your ISO amount decrease also the quality of the image. Noise/ Grain/ Specs will start to appear in some of the lower camera  models at higher ISO settings and less in high end camera models. Here are some example of images taken in ISO 100 and 800 respectively.

Fig. 1 ISO setting 100 (more available light less ISO required)

Fig. 2 ISO setting 800 (less available light high ISO required)

Photographers adjust their ISO settings based on aperture/ shutter speed demand. To be able to fully understand this, get your camera and shoot inside your room with only the window as the source of light and outside of your house for comparison. Choose the manual mode, let's say you want an aperture value of f5.6, shutter speed of 1/125sec and ISO 100. Put your eyes in the viewfinder, are the exposure bars mostly in the left (underexposed) or right(overexposed)? And you don't want to  adjust your aperture and shutter speed settings to get a properly exposed image. Start increasing your ISO see what happens to your exposure bars. Adjust it until you have it in the middle or 0. You can also do this with aperture, shutter speed and programmed auto mode. If you want to shoot at a lower ISO settings to get a better image quality, you can adjust your aperture (depth of field/ subject sharpness from background) and shutter speed(motion blur/ free or show motion) settings.