ISO is a measure of film "speed" or sensor sensitivity. ISO stands for International Organization for Standardization and replaced what used to be called ASA, which was American Standards Association. Both mean the same thing as far as photography is concerned. Think of higher the ISO number, the more light you are adding to the image. 

ISO goes hand in hand with shutter speed and aperture. Every time I'm indoors and it's dark, I crank my ISO's to 6400 and sometimes turn my flash on if it's really dark. I keep my shutter speed around I/200 of a second and my aperture between 3.2 and 4.0. The higher the ISO, the more grain your images will produce. This can be corrected later in post production by using Adobe Lightroom.

When I'm outdoors in the shade my ISO is 400 and my shutter speed is 400 and my aperture is 4.0.

When I'm outside in the sun my ISO is 100 and my shutter speed is 1000 and higher. Depending on what you're photographing your aperture can be whatever you'd like. The same goes for all of these setting. They are just a starting point and then make minor tweaks here and there.

Hopefully these tips will help you in manual mode. Feel free to contact me here if you have any questions.