I assume you are talking about a retinal scan, where they take a picture of the back of your eye to check for damage due to diabetes.
The procedure can vary slightly. Sometimes – or perhaps with some cameras – you don't need drops to dilate the pupils. But most people I know do actually get a drop in each eye so the camera can "see in" properly. The drops may be a little stingy for a few moments, but no big deal.
If you get the drops you'll wait for maybe 15 minutes for them to take effect. Then you sit in front of a machine – the type of thing you'd expect in an opticians, where you rest your chin on a pad and look through lenses. You just look at a little light and they take a picture of the back of your eye. No pain, no fuss, no hassle.
Usually these days it's a digital picture so you may get to see, on screen, the network of blood vessels at the back of the eye. Really interesting.
If the drops have been used to dilate the pupils, be warned that even a cloudy day will seem unpleasantly bright when you go outside, so take a pair of dark glasses. You can't drive right away if you've had the drops.
The worst of the brightness passes within an hour or so, and everything's back to normal vision in a couple of hours.
So don't worry about it at all.