Apature or F Stop of camera lens
larger number = smaller opening on left

smaller number = larger opening on right

The F Stop and Shutter settings have to work together to obtain the correct light exposure
your light meter can calculate this for you.
But the light meter may give you false readings in certain types of light
Bright days with snow for instance, cloudy conditons. people under canopys or in shade, so you have to be aware of what your photograhing and make changes to account for it.

Here is an example, your in bright sunlight and two people are sitting on a picnic table under an awning, you focus, and make changes to your apature F Stop and to your shutter speed till the light meter shows you your right on the dot...Wrong

The light meter just lied to you not knowing it did so, remember it is a machine, it is looking at the entire area of your photograph and sees bright light all about, but isn't taking into account the darker faces under the canopy, thus you get a photograph of their outline instead of their faces, bet you have done that before and didn't know the problem with a point and shoot camera, the SLR allows you to change it and make ajustment for a perfect photo.

In that case, simply walk up close I mean within say a foot and take your meter reading off on of their faces, do your settings according, then walk back out in the sun and take the photo, the meter will tell you that it is over exposed, and it is right you are, but the over exposure will allow the faces of the people to be burned in longer giving them the necessary detail and the rest of the photo will be just fine, try it, trust me you will now get that difficult photo using this tecnique!

However you could go bye a Gossen light meter which is accurate to 1 percent for $600 dollars and it will tell you the setting, my way is cheaper with the same results. I'd still love to have the German made meter, I've used one, it is wonderful you get every shot everytime no guess work.