Hello there!
Aperture is defined as being the size of the opening in the lens. It can be set automatically or manually. If you have a small aperture, less light will reach the camera, while a larger aperture will let in more light. So, the larger the aperture in a sunrise/sunset photo, the brighter it will be.
Aperture is measured in f-stops.
The recommended aperture for sunrise/sunset is f-8, f-11, or even higher to help minimise the amount of light that gets into the camera to prevent the image from becoming too bright.
Hope this helps!
-Astro, Helper-In-Training