Don't worry, you're not the only one who doesn't already know!
Officially we are described as 'a person who leads a ceremony such as a marriage or a funeral' (Cambridge Dictionary)
Well, yes, that is very true. But there is so much more to it than that. For one, we offer plenty more types of service. Whenever there is an occasion that needs marking, a celebrant can do that.
Lots of people have faith and these special occasions are often marked within their own favoured religious environments. However, many folks don't consider themselves as religious at all but still feel it is important to commemorate life's big events. Children can be welcomed into the world and have their name given to them in a more secular way, family and friends can make promises to that child all whilst remaining true to the family's beliefs.
That isn't to say that there is no spirituality involved in a celebrant led ceremony. The content is entirely based on the family's wishes.