A picture on a digital camera is taken by the camera sensor that is open to light for a short period of time.
So tree building block of the calculating of the exposure are aperture, shutter speed and ISO. There is a meter inside of any digital camera who measure the light reflected by the scene and base of the opening of the aperture and ISO set the shutter speed accordingly.
The bigger the aperture (or the smaller the f number) is the more light reach the camera sensor and the smaller shutter speed is needed for that that picture.
The bigger the ISO number is (sensitivity of the sensor to the light ) the faster shutter speed will be.
Here are few aspects to consider:
-double the ISO will double the shutter speed
-the higher the ISO the more noiser signal will be output from camera sensor
-shutter speed have to be least 1/125 for hand held photography ( if you are carefully with the camera shake you can bring the shutter speed to 1/100 or even 1/50)
All these settings are being took care by the camera itself, but if you understand them and you have a a camera that permit to change them, you can start making better picture.