It is one of the most important things about telescopes
An Aperture is an opening. And in a telescope aperture is the main opening where all the light comes in. And it really is the single most important characteristic of a scope. Because the bigger the aperture the more a telescope can see.
Let me give you an example of aperture and how it works.
Let's say it is raining outside and you put a cup out in this rain. How much water would it gather? And let's say you put a 50 gallon drum out in the rain. Would it gather more rain? Yes it would because the aperture (or opening) of the drum is larger than the aperture of a cup. So, it gathers in more rain.
The same thing applies to a telescope. The larger the aperture the more light that it will gather. So, the most important aspect of a telescope is the aperture. Bigger aperture means more light. This is why the pursuit of astronomy has always been about trying to make larger and larger telescopes!
And this is why telescopes are almost always named or described by their aperture size. We have six inch telescopes, eight inch telescopes, ten inch telescopes, 60 millimeter telescopes, 70 millimeter telescopes etc.
When Buying A telescope - So when you are buying a telescope give some thought to the aperture. It is very important. And of course the larger this dimension the more expensive the telescope will be.