A consultation is a short appointment with no treatment in it. You talk about what you want, the professional looks at your hair, skin or nails, and you both find out whether the thing you have in mind is achievable, how long it will take and what it will cost.
Book one before anything major or irreversible: a big colour change, extensions, a first appointment with a new professional after a bad experience, or anything you have only ever seen in a photograph. It is the single most effective way to avoid a disappointing result, because most disappointing results are a mismatch of expectations rather than bad work.
Consultations are often free and always short. Some studios require one before they will take a booking for certain services, and some use it to run a patch test at the same time, which many colour services need at least a day in advance.
Bring photographs of what you want and, if you can, of your hair's history — box colour, bleach, a previous correction. A professional can see the current state; they cannot see what was done to it two years ago, and that is often what decides the plan.