Symbols are labels or signs that that are used to express meaning.  It is arbitrary in the sense that it works because of an agreement among a group of people as to what the symbol represents (Beck et al., 2005). 

Example:

This is a picture of a tree.  The actual tree within this picture is a symbol as it represents a type of plant that has a trunk, grows to a considerable height, and often develops branches near its top. The meaning of the symbol for what we know to be a "tree" is deduced with the help of signs.  Thus, the word "tree" and its symbol allows you to associate this tall plant with the idea it represents. 

See also the definition of signs.