A compound microscope has two sets of lenses, the eyepiece lens and the objective lens. The sample to be studied is kept in a slide that is placed on a flat platform referred to as a stage. The system of lenses close to the slide is known as the objective and the one on top of the microscope is known as the eyepiece.
To illuminate the sample being viewed it is possible to use a mirror that reflects light off an external source onto the slide. It is also possible to have an independent source of light that is used for illumination. An opening in the stage on which the slide is placed allows light to pass through it and light up the slide. It is possible to control the amount of light that shines through the slide by changing the size of the opening present in the stage. It is also possible to change the illumination by moving the mirror in microscopes that use one or by changing the brightness of the light source in the Illuminator.
The part as known the opening in the stage in a compound microscope, namely adjustable diaphragm is used to adjust the amount of light that shines through the slide.
According to funtriva.com, the piece that allows you to adjust the amount of light that's coming through the microscope is called the adjustable diaphragm. It is located under to stage (where what you are observing is placed on) and can be rotated to make the light intensity change. If you have a low magnitude lenses on your microscope, you will need less light.
In laymen's terms,
The adjustable diaphragm is located under the stage, and you can rotate it to turn the light on, off, and brighter and darker. The lower the magnitude of the microscope lenses, the lower the light needs to be, and the higher the magnitude, the higher the light.