what is digital camera aperture?

Answer 1

Low numbers, like f/2, are for taking pictures in low light and also for creating blurry backgrounds. Great for portraits and such.
High numbers, like f/11, are for getting everything in focus, but also require lots of light. You'd use that for landscapes.
With point and shoots, the aperture is usually described as a range, say f/2.8-4.5. What those numbers mean, is that you can set the aperture a low as 2.8 for wide angle shots, but only as low as 4.5 when you zoom all the way in. You can usually set the aperture to a high of around 8. The upper limit is not affected by how much you zoom in or out.
Advanced point and shoots will have an aperture priority mode, so you can experiment with this and take more creative control over your shots. But because of the tiny sensors in point and shoots, the effect is never quite a pronounced as you'd like it to be. With digital SLR cameras (and the bigger sensors), aperture becomes much more important.

Answer 2

An aperture is the lens opening. Digital or single lens reflex all cameras need an aperture to allow light in and the image to form on film or exposure of your image digitally.