Aperture is a measure of the size of the camera shutter (the hole letting in light to the camera sensor). A bigger opening let’s in more light and a smaller less light.
The aperture is stated as a fraction number, like f/1.8, f/5.6 or f/22. Often the “f/“ part is omitted, leaving for instance 1.8, 5.6 and 22.
A larger aperture means a smaller opening and a smaller aperture means the opening is larger. If you are shooting in low low light conditions you usually have to let in more light to the sensor. This can be done either by using a smaller aperture (larger opening) or by using a slow shutter speed (or both).The opposite is true when there is a lot of light.
Aperture can also be used to manipulate depth of field (DOF). A large opening (low aperture number) creates a shorter depth of field and a smaller opening creates a longer depth of field. A short depth of field is often used for portraits where a blurry fore- and background is appreciated. A long depth of field is used for architecture where as much detail as possible should be in focus.
In phone cameras, the aperture is usually fixed to minimize moving parts and size.