During this part of your eye test, the optometrist will check how far off to the sides you can see when looking straight ahead. This can be done as simply as wiggling their fingers and asking if you see them, provided there is nothing else happening with your eyesight.
It can also be quantified, by using a piece of equipment called a visual field scanner. These, again, are all slightly different, but work in a similar way. They use pictures or lights, and these flash in your peripheral vision. You will be asked to click a button every time you see one, and a plot can be produced to highlight any you may have missed.