Aperture is simply the diameter of the hole at the front of the telescope. It's measured in millimeters or inches. The bigger the number, the bigger the hole. The smallest aperture available today is 1 inch. You can find out what aperture you need by looking up your desired magnification. Magnification is just another word for zoom.
When you look at the night sky, you'll notice that the further away an object is, the dimmer it appears. That's because light travels faster when traveling farther. So if you have a small telescope, you won't be able to see very distant objects. To compensate for this effect, astronomers use a technique called "magnification". They magnify the image so that they can see things that are farther away.
For instance, let's say you're using a 20x telescope. If you were to look at Jupiter, which is over 500 million miles away, you would not be able to make out any details. But if you used a 200x telescope, you could easily see Jupiter's moons.