An aperture isa value expressed as a factor (f) number and indicates how much of the physical size of the lens is being used to control the amount of light reaching the sensor. The size of the aperture is shown as a seemingly unhelpful set of numbers; the basic ones are:-

f2.8: f4: f5.6: f8: f11: f16: & f22

There is a technical relationship between these numbers and a lens, but that need not concern the novice. However, f2.8 is the largest aperture allowing more light to reach the digital sensor, whereas f22 is the smallest and as you proceed across the scale the amount of light reaching the sensor is halved at each stage.

This has been at the bedrock of photography for years and it still applies today even with the most sophisticated computerised camera. The correct combination of a shutter-speed and aperture will always produce a perfectly exposed image, but computerised cameras also show the factor values of apertures in between such as f6.3. This is useful for the sake of accuracy, but confusing when trying to understand the principle!

A lens with f2.8 as its largest aperture is capable of allowing more light to reach the sensor than an f4 lens and therefore better at taking shots in low light without flash. But don't forget that a shutter-speed which determines how long the aperture remains open also contributes to the equation that controls the amount of light reaching the sensor for a correctly exposed image.

Note: The smallest aperture available on compact and bridge cameras is usually f8.