You may have heard of the Sunny 16 rule that many photographers use as a guideline for exposure. This rule can be used as a good initial starting that can then be tweaked for accuracy.
The Sunny 16 rule suggests that on a sunny day if you set your aperture to F-16, and then the ISO and shutter speed to the same value (actually shutter speed is the reciprocal to the ISO) that you will get a good exposure. For example, on a sunny day if you set the aperture to F-16 and the shutter speed to 1/400 with an ISO of 400 it should yield you a perfect exposure. Of course, this depends on the circumstances. Is the sun directly overhead, or at an angle? Is it to your back, or to the side? These will have an effect on the exposure and should be considered. Bracketing could help nail your exposure even more.
So what do you do if you don't want to shoot at F-16? Just set it to the value you want and then adjust one or both of the other two variables (shutter speed and ISO). Let's say you want to shoot at F2.8. What can you do? Well changing the aperture from F-16 to F2.8 is letting in 5 more stops of light. So to compensate for this you will need to adjust ISO and/or shutter speed to reduce the light by 5 stops. Most cameras will not go below 100 ISO, and if they can you usually don't want to go there. In our example going from ISO 400 to ISO 100 is only 2 stops. That means if we reduce the ISO by these 2 stops we will still need to reduce the shutter speed by 3 stops. This would result in a shutter speed of 1/1600. So our final settings would be a shutter speed of 1/1600, an ISO of 100, and an aperture of 2.8. These settings would yield the same exposure as the Sunny 16 rule.
With today's cameras this rule is not as important as it use to be, but it's still good to know. Especially if you're shooting manual. Most of the time you can just determine a couple of the variables and then dial in the third until your meter shows a good exposure. Of course, that's reflective metering and that's another subject. But then the Sunny 16 rule is just an estimate as well. Shooting in one of the automatic modes will also help by automatically adjusting one of the other variables (usually not ISO). Try changing your camera mode to Aperture Priority with the aperture set at F-16 and then shooting on a sunny day. See how close the shutter speed and ISO are to the same value. They should be pretty close.
Hope this has helped you understand what all the fuss is about with the Sunny 16 rule. Happy shooting.