Symbols stand for other things in an arbitrary fashion. The word "dog" is a symbol for the animal. There is nothing about the sounds that makes it mean "dog" The word for dog differs from one language to another. The foot print of a dog, is not a symbol since it has an intimate connection with the animal and hense is not aribitrary.
Some symbols may be rather universal, others cultural and others may be constructed by an individual. The west as a place associated with death occurs in many places and is understandable since the west is where the sun, moon, stars and planets all set. Other symbols like Santa Clause, are cultural and are the product of a given culture. Individual symbols are constructed and in order to understand them one must have sufficient context for them to allow intelligibility.
Films which are rich in symbols are often called "think" or "opaque.
The decipherment of a text ()like a film) means breaking the coded system in which and through which the films makers sends the messge. The process is called hermeneutics. Technically, deciphering is used to break a cipher which is a way to hide a messge by replacing one letter with another. Decoding techinically means to break a code which is a system of hiding a message by replacing a word with another. So in a cipher one might write "dpef" for "code" - lower each letter by 1 d=>c; p=>o; e=>d and f=>e. If however I write. "I saw a whale" and it means "I saw a destroyer" (i.e. substitute "destroyer" for "whale") then I have a code.
2001: A Space Odyssey uses some individual symbols and hence is a difficult film to "read".
Click 2001 for more information on the film.