The Director of a film (or a play, or a television show episode) is the person responsible for the creative vision of the piece.
They create a concept from the script (which may or may not be something concretely found in the script, it may be metaphorical or tangential) and from the concept lead the design and production team towards a collaborative vision.
Once rehearsals/filming have begin, the director blocks the piece (i.e. tells actors where to move), provides objective and subtextual support to the actors (i.e. tells them why they are saying the things the writer wrote) and ensures that the visual style and setting are within the original vision or concept parameters.