That means that they will be putting some drops in your eyes. these drops will make you pupil (the black hole in the middle of the iris
) larger. this allows the person testing your eyes to have a clearer view of the back of your eyes to check for any eye problems.
it also allows them to make a more accurate measurement of the prescription in your glasses.
There are a couple of side effects of these drops too. Its will make your vision a little blurred for a few hours and it will also make you very light sensitive. make sure you take some sunglasses with you to protect your eyes from the bright lights afterwards.
all these side effects are temoporary and only last for a few hours.
nothing to worry about. its a very common and safe procedure