don't worry - it's easy and painless.....they want to check your peripheral vision. you will look into a lens (like a kalaidascope) and look for a light and you will press a button....you will have a clicker in each hand and when the light is on the right, you click the button in your right hand, and when you see the light on the left, you click the button in your left hand.....
There are two types of visual field tests-the automated one Rose told you about and another one run by a person. I prefer the person test because in the automated one the light spot appears anywhere at random. It blinks and disappears. In the person-run test, the light is operated by the assistant and it moves in straight lines. That is, the spot of light will start outside your peripheral vision and slowly enter it until you click. Then it will start at another spot and travel in a line toward the centre of the eye until you notice it. It doesn't just pop up anywhere in your range of vision.
At any rate, they are both a piece of cake and I hope you have good results.