An ISO in digital photography measures the sensitivity of the image sensor.
The lower the number the darker the image, the higher the number the lighter the image. Some photographers use an higher ISO when shooting in dark situations to get faster Shutter speeds. 'Speed' is the time it takes for the light of what you're shooting to reach your camera's sensor. For example:
ISO 100 would take 1 second to reach your camera's sensor
ISO 200 - 1/2 of a second
ISO 400 - 1/4 of a second
ISO 800 - 1/8 of a second
ISO 1600 - 1/16 of a second
ISO 3200 - 1/32 of a second
The longer the light takes the noiser the image. The quicker the light takes to get to the sensor the cleaner the image.
When shooting, you really shouldn't go any higher than 200 ISO. If you need to make the subject you're shooting brighter I would add a little more fill light, or just rethink the camera angle. When taking a picture in low light you tend to get an image with a lot of grain or as it's called "Noise." Unless you're going for that look, it really won't give you the sharpest image.
Tell me what you shot today using an ISO at 200