The aperture is the opening or hole into the camera. It allows light in through the lens. It can be a smaller opening or larger. We measure it in fractions that are called f-numbers.
The f-number is almost always identified by the f-first.
Larger apertures allow more light in and are shown as a lower f-number. Smaller apertures allow less light in and are shown as a larger f-number.
Aperture also controls the depth of field in an image. The first thing to understand is that everything in the picture is what is called "the field". The field has a vertical and horizontal measure.
The field also has a depth measure - a measurement of how far from the lens away the field goes. The depth of the field is the parts that are in focus/are clear. It can be everything or only part.
This image has everything in focus from the first point in front of the camera to everything stretching into infinity. This is called WIDE DEPTH OF FIELD
This image has a mid-depth of field. Some parts are in focus and some are not.
This image has a shallow depth of field - that is, only one area of the field is in focus and nothing else is.
Wide depth of field is achieved with very small apertures (very high f-numbers like f-22 to f-32); shallow depth of field is achieved with very large apertures (low f-numbers like f-5.6 to f-1.2).
To achieve shallow depth of field there must be something very near the lens (near field), something a bit away from the lens (mid-field) and something far away (far field).