Large aperture (f/2.0, f/2.8 etc.) lets in more light to the camera shutter for an exposure, while small aperture (f/11, f/16, f/22 etc.) has a smaller opening in the lens diaphragm to let in less light for a given exposure.
Shutter Speed
The Shutter Speed setting controls how long the shutter opens to expose the image sensor to that light. Open it for just a millisecond and you can freeze a fast moving subject.
ISO
On a bright sunny day too much light hitting the sensor can cause an overexposure so make it less sensitive with a low ISO number. If your shooting conditions are dark the sensor needs to detect more light so increase its sensitivity with a higher ISO.