ISO – International Standards Organization. This is basically the standard measurement for how sensitive the film or image sensor. Ranging from about 100 to 3200 the 100 is not sensitive to light and better for very bright settings such as a sunny beach, where as a 3200 would come in handy for extremely dim environments.
When I began with film 400 ISO was the standard. It works well both indoors and outside generally. It is also closer to the low end of the scale, since high ISO’s tend to produce a lot of grain/noise.
One way to help get to a lower ISO without sacrificing other settings, would be to put it onto a tripod and have a slower shutter speed, thereby allowing more light without any camera shake. You can also shoot with a flash to get quicker shots.
The light meter refers to the sensor within the camera, and is read by the exposure meter. The camera aims for 18% gray. Keep in mind there are different ways to do this, center-weighted, partial, and spot metering.
In order to move the arrow, showing where your current settings will put you on the scale you will need to adjust either your shutter speed or ISO.
Think of the middle target as a guide. I tend to slightly over-expose my images which would be towards the positive end of the scale. Of course, dark and moody shots are great too in which you would move the arrow to the negative side of the scale.
There are a number of ways to achieve the perfectly exposed image, with these two tools. Some prefer handheld light meters to be exact.