It is used for focusing.
Coarse adjustment
you turn the fine and coarse adjustment knobs
There are two adjustment knobs (coarse and fine) on a microscope so you can move the stage at two different speeds. The coarse moves a lot per revolution, while the fine adjustment knob moves the stage at such small increments that it is nearly impossible to notice when looking from the side. The coarse adjustment knob should be used to get the stage closer to the lens only while you are looking at the microscope from the side. The fine adjustment knob can be used when you are looking into the microscope because there is a much lower chance of running the stage into the lens and breaking it.
Microscopes typically have knobs for adjusting the focus (coarse and fine adjustment knobs), magnification levels (objective lens turret), and the stage (stage control knobs for moving the specimen). Other common knobs include the condenser adjustment knob for controlling the amount of light passing through the specimen and the mechanical stage knobs for precise movement of the specimen.
mechanical stage adjustment knobs
The coarse adjustment knob is used to make large adjustments to the focus of the microscope, bringing the specimen into view. The fine adjustment knob is used for precise focusing by making small adjustments to the sharpness of the image. The combination of both knobs allows for clear and accurate visualization of the specimen.
the big knob is called the coarse adjustment knob and the smaller one is the fine adjustment knob coarse does a large scale focus, fine does such small scale focus you can only see a difference in how the (item under microscope) looks. You don't see the stage / nosepeice move up or down.
Coarse adjustment knob is used for initial rough focusing by moving the lenses further or closer to the specimen. Fine adjustment knob is used for fine-tuning the focus for sharper clarity by making small adjustments to the position of the lenses.
Coarse adjustment of what?
It is the first knob you should turn to bring the object into rough focus, before tinkering with the finer tuning knobs.
To focus a microscope using the coarse adjustment knob, gently turn it in one direction until the specimen comes into view. Then, use the fine adjustment knob to further sharpen the image. Avoid using excessive force on either knob to prevent damaging the microscope or specimen.
In a compound microscope, the body tube is moved up and down for focusing by turning the coarse adjustment knob. The stage remains stationary during this process. The coarse adjustment knob controls the position of the body tube, which contains the objective lens, to bring the specimen into focus.