The automatic perimetry is the reference standard exam to assess visual function.
Basically you will press a button everytime you see a light stimulus presented at various locations of your visual field: some at central region, some at the periphery. Stimuli will be presented at different intensities, from very bright to very faint. It is important to keep your eyes still, fixated on a central target throughout the exam, and to just press the button when you do notice the stimulus.
It is a painless procedure, but it will take some minutes, and it can be considered as a burdensome and/or boring procedure. However, it is very important that you maintain your attention during the whole examination and perform this test in the best way possible, as the obtained information is essential to clinical management decisions. Thus, keep calm and focused, as this is not a pass or fail exam, it is just to check how your vision is doing to ensure appropriate treatment.
Glaucoma is associated with characteristic visual field loss, which has to be monitored at least once or twice a year to establish if there is any progression of disease. As a subjective exam, visual field results do vary/fluctuate, and repeating VF exams may be considered a common procedure to confirm glaucoma progression.