What is aperature in a camera?

Aperture is the size of the hole that light has to pass through to get to the photo paper. An aperture with a small number would be a big whole and vice versa.

In practice: If the shutter speed was high (short) the aperture would be low, meaning a large hole, so that the amount of light hitting the photo paper was correct.

If the shutter speed was longer, the aperture would be a high, meaning a small hole, so that the correct amount of light met the paper... and not too much or too little.

It is also how you control the depth of field in your photograph. Small aperture gives you a deep depth of field, and a large gives you an area of sharp focus.

Hope that's slightly helpful!