When a new film is made, it has to be advertised like any other new product, to let people know it exists and to encourage them to go to the cinema to see it.
The advertising of a film is known as film promotion or film marketing and the people who are responsible for this are the distribution company, so–called because they distribute (give out) the films to the cinemas and distribute the promotional material around the country.
The way in which a film is promoted can have a huge effect on whether or not it is successful. Films are expensive to make and if the public do not buy tickets at the box office to see the film, a lot of money will be lost.