With some machines, you have to put your chin on a chin-rest. Some tests are more like games. In one test, the optician puffs some air into your eyes quickly. The optician has a letter chart on the wall. It doesn't matter if you don't know your letters.
Some opticians have pictures instead.
The optician will ask you about what you can see on the charts. The optician may put a special glasses frame on your nose and put different lenses into it. They will ask you which lenses help you see best. Don't worry if you can't answer. Lots of people find this difficult.