The aperture is a mechanism on the lens that regulates the amount of light falling through the lens. You can think of it as a simple hole, the size of which can be adjusted either directly on the lens or on the camera.
The aperture in photography is an iris diaphragm.
Together with the exposure time and the light sensitivity (ISO), the aperture determines the exposure of the photo, the two being in direct proportion to each other. If I choose a larger aperture (the hole enlarges) I have to shorten the exposure time at the same time to get a correct exposure.
Furthermore, the aperture largely determines the depth of field: a large aperture produces a shallow depth of field, a small aperture a great depth of field.